diff --git a/src/DayPicker.tsx b/src/DayPicker.tsx
index 8a176bb14..5c5e6f9bb 100644
--- a/src/DayPicker.tsx
+++ b/src/DayPicker.tsx
@@ -11,7 +11,6 @@ import type { CalendarDay } from "./classes/CalendarDay.js";
 import { getClassNamesForModifiers } from "./helpers/getClassNamesForModifiers.js";
 import { getComponents } from "./helpers/getComponents.js";
 import { getDataAttributes } from "./helpers/getDataAttributes.js";
-import { getDateLib } from "./helpers/getDateLib.js";
 import { getDefaultClassNames } from "./helpers/getDefaultClassNames.js";
 import { getFormatters } from "./helpers/getFormatters.js";
 import { getMonthOptions } from "./helpers/getMonthOptions.js";
@@ -19,8 +18,8 @@ import { getStyleForModifiers } from "./helpers/getStyleForModifiers.js";
 import { getWeekdays } from "./helpers/getWeekdays.js";
 import { getYearOptions } from "./helpers/getYearOptions.js";
 import * as defaultLabels from "./labels/index.js";
-import type { FormatOptions, LabelOptions } from "./lib/dateLib.js";
-import { enUS } from "./lib/locales.js";
+import { DateLib } from "./lib/dateLib.js";
+import { defaultLocale } from "./lib/locales.js";
 import type {
   DayPickerProps,
   Modifiers,
@@ -45,28 +44,43 @@ import { isDateRange } from "./utils/typeguards.js";
  */
 export function DayPicker(props: DayPickerProps) {
   const { components, formatters, labels, dateLib, locale, classNames } =
-    useMemo(
-      () => ({
-        dateLib: getDateLib(props.dateLib),
+    useMemo(() => {
+      const locale = { ...defaultLocale, ...props.locale };
+
+      const dateLib = new DateLib(
+        {
+          locale,
+          weekStartsOn: props.weekStartsOn,
+          firstWeekContainsDate: props.firstWeekContainsDate,
+          useAdditionalWeekYearTokens: props.useAdditionalWeekYearTokens,
+          useAdditionalDayOfYearTokens: props.useAdditionalDayOfYearTokens
+        },
+        props.dateLib
+      );
+
+      return {
+        dateLib,
         components: getComponents(props.components),
         formatters: getFormatters(props.formatters),
         labels: { ...defaultLabels, ...props.labels },
-        locale: { ...enUS, ...props.locale },
+        locale,
         classNames: { ...getDefaultClassNames(), ...props.classNames }
-      }),
-      [
-        props.classNames,
-        props.components,
-        props.dateLib,
-        props.formatters,
-        props.labels,
-        props.locale
-      ]
-    );
+      };
+    }, [
+      props.classNames,
+      props.components,
+      props.dateLib,
+      props.firstWeekContainsDate,
+      props.formatters,
+      props.labels,
+      props.locale,
+      props.useAdditionalDayOfYearTokens,
+      props.useAdditionalWeekYearTokens,
+      props.weekStartsOn
+    ]);
 
   const {
     captionLayout,
-    firstWeekContainsDate,
     mode,
     onDayBlur,
     onDayClick,
@@ -77,22 +91,9 @@ export function DayPicker(props: DayPickerProps) {
     onNextClick,
     onPrevClick,
     showWeekNumber,
-    styles,
-    useAdditionalDayOfYearTokens,
-    useAdditionalWeekYearTokens,
-    weekStartsOn
+    styles
   } = props;
 
-  const formatOptions: FormatOptions = {
-    locale,
-    weekStartsOn,
-    firstWeekContainsDate,
-    useAdditionalWeekYearTokens,
-    useAdditionalDayOfYearTokens
-  };
-
-  const labelOptions: LabelOptions = formatOptions;
-
   const {
     formatCaption,
     formatDay,
@@ -144,15 +145,8 @@ export function DayPicker(props: DayPickerProps) {
   } = labels;
 
   const weekdays = useMemo(
-    () =>
-      getWeekdays(
-        locale,
-        props.weekStartsOn,
-        props.ISOWeek,
-        props.timeZone,
-        dateLib
-      ),
-    [dateLib, locale, props.ISOWeek, props.timeZone, props.weekStartsOn]
+    () => getWeekdays(dateLib, props.ISOWeek, props.timeZone),
+    [dateLib, props.ISOWeek, props.timeZone]
   );
 
   const isInteractive = mode !== undefined || onDayClick !== undefined;
@@ -317,7 +311,6 @@ export function DayPicker(props: DayPickerProps) {
               navStart,
               navEnd,
               formatters,
-              locale,
               dateLib
             );
 
@@ -373,7 +366,7 @@ export function DayPicker(props: DayPickerProps) {
                       captionLayout === "dropdown-years" ? (
                         <components.YearsDropdown
                           className={classNames[UI.YearsDropdown]}
-                          aria-label={labelYearDropdown(labelOptions)}
+                          aria-label={labelYearDropdown(dateLib.options)}
                           classNames={classNames}
                           components={components}
                           disabled={Boolean(props.disableNavigation)}
@@ -396,7 +389,7 @@ export function DayPicker(props: DayPickerProps) {
                     >
                       {formatCaption(
                         calendarMonth.date,
-                        formatOptions,
+                        dateLib.options,
                         dateLib
                       )}
                     </components.CaptionLabel>
@@ -406,7 +399,7 @@ export function DayPicker(props: DayPickerProps) {
                   role="grid"
                   aria-multiselectable={mode === "multiple" || mode === "range"}
                   aria-label={
-                    labelGrid(calendarMonth.date, labelOptions, dateLib) ||
+                    labelGrid(calendarMonth.date, dateLib.options, dateLib) ||
                     undefined
                   }
                   className={classNames[UI.MonthGrid]}
@@ -419,7 +412,7 @@ export function DayPicker(props: DayPickerProps) {
                     >
                       {showWeekNumber && (
                         <components.WeekNumberHeader
-                          aria-label={labelWeekNumberHeader(labelOptions)}
+                          aria-label={labelWeekNumberHeader(dateLib.options)}
                           className={classNames[UI.WeekNumberHeader]}
                           style={styles?.[UI.WeekNumberHeader]}
                           scope="col"
@@ -431,7 +424,7 @@ export function DayPicker(props: DayPickerProps) {
                         <components.Weekday
                           aria-label={labelWeekday(
                             weekday,
-                            labelOptions,
+                            dateLib.options,
                             dateLib
                           )}
                           className={classNames[UI.Weekday]}
@@ -439,7 +432,7 @@ export function DayPicker(props: DayPickerProps) {
                           style={styles?.[UI.Weekday]}
                           scope="col"
                         >
-                          {formatWeekdayName(weekday, formatOptions, dateLib)}
+                          {formatWeekdayName(weekday, dateLib.options, dateLib)}
                         </components.Weekday>
                       ))}
                     </components.Weekdays>
@@ -515,7 +508,7 @@ export function DayPicker(props: DayPickerProps) {
                               ? labelGridcell(
                                   date,
                                   modifiers,
-                                  labelOptions,
+                                  dateLib.options,
                                   dateLib
                                 )
                               : undefined;
@@ -555,7 +548,7 @@ export function DayPicker(props: DayPickerProps) {
                                     aria-label={labelDayButton(
                                       date,
                                       modifiers,
-                                      labelOptions,
+                                      dateLib.options,
                                       dateLib
                                     )}
                                     onClick={handleDayClick(day, modifiers)}
@@ -571,10 +564,10 @@ export function DayPicker(props: DayPickerProps) {
                                       modifiers
                                     )}
                                   >
-                                    {formatDay(date, formatOptions, dateLib)}
+                                    {formatDay(date, dateLib.options, dateLib)}
                                   </components.DayButton>
                                 ) : (
-                                  formatDay(day.date, formatOptions, dateLib)
+                                  formatDay(day.date, dateLib.options, dateLib)
                                 )}
                               </components.Day>
                             );
diff --git a/src/classes/CalendarDay.ts b/src/classes/CalendarDay.ts
index 86eb54359..ff3455554 100644
--- a/src/classes/CalendarDay.ts
+++ b/src/classes/CalendarDay.ts
@@ -1,5 +1,4 @@
-import type { DateLib } from "../lib/index.js";
-import { dateLib as defaultDateLib } from "../lib/index.js";
+import { DateLib } from "../lib/dateLib.js";
 
 /**
  * Represent the day displayed in the calendar.
@@ -13,7 +12,7 @@ export class CalendarDay {
     date: Date,
     displayMonth: Date,
     /** @ignore */
-    dateLib: DateLib = defaultDateLib
+    dateLib: DateLib = new DateLib()
   ) {
     this.date = date;
     this.displayMonth = displayMonth;
diff --git a/src/formatters/formatCaption.test.ts b/src/formatters/formatCaption.test.ts
index 2606958ac..5f2edc67d 100644
--- a/src/formatters/formatCaption.test.ts
+++ b/src/formatters/formatCaption.test.ts
@@ -1,18 +1,27 @@
 import { es } from "date-fns/locale/es";
 
-import { dateLib } from "../lib";
+import { DateLib } from "../lib";
+import { defaultLocale } from "../lib/locales.js";
 
 import { formatCaption } from "./formatCaption";
 
 const date = new Date(2022, 10, 21);
 
 test("should return the formatted caption", () => {
-  expect(formatCaption(date, {}, dateLib)).toEqual("November 2022");
+  expect(
+    formatCaption(date, {}, new DateLib({ locale: defaultLocale }))
+  ).toEqual("November 2022");
 });
 
 describe("when a locale is passed in", () => {
   test("should format using the locale", () => {
-    expect(formatCaption(date, { locale: es }, dateLib)).toEqual(
+    expect(formatCaption(date, { locale: es })).toEqual("noviembre 2022");
+  });
+});
+
+describe("when a locale is passed in through date lib", () => {
+  test("should format using the locale", () => {
+    expect(formatCaption(date, {}, new DateLib({ locale: es }))).toEqual(
       "noviembre 2022"
     );
   });
diff --git a/src/formatters/formatCaption.ts b/src/formatters/formatCaption.ts
index edba56990..42e3a7c07 100644
--- a/src/formatters/formatCaption.ts
+++ b/src/formatters/formatCaption.ts
@@ -1,8 +1,4 @@
-import {
-  FormatOptions,
-  dateLib as defaultDateLib,
-  type DateLib
-} from "../lib/index.js";
+import { DateLib, type FormatOptions } from "../lib/dateLib.js";
 
 /**
  * Format the caption of the month.
@@ -15,9 +11,9 @@ export function formatCaption(
   month: Date,
   options?: FormatOptions,
   /** @ignore */
-  dateLib: DateLib = defaultDateLib
+  dateLib: DateLib = DateLib.fromOptionsDefaultLocale(options)
 ) {
-  return dateLib.format(month, "LLLL y", options);
+  return dateLib.format(month, "LLLL y");
 }
 
 /**
diff --git a/src/formatters/formatDay.ts b/src/formatters/formatDay.ts
index 858f14b2c..82072dd04 100644
--- a/src/formatters/formatDay.ts
+++ b/src/formatters/formatDay.ts
@@ -1,5 +1,4 @@
-import type { FormatOptions, DateLib } from "../lib/dateLib.js";
-import { dateLib as defaultDateLib } from "../lib/index.js";
+import { DateLib, type FormatOptions } from "../lib/dateLib.js";
 
 /**
  * Format the day date shown in the day cell.
@@ -12,7 +11,7 @@ export function formatDay(
   date: Date,
   options?: FormatOptions,
   /** @ignore */
-  dateLib: DateLib = defaultDateLib
+  dateLib: DateLib = DateLib.fromOptionsDefaultLocale(options)
 ) {
-  return dateLib.format(date, "d", options);
+  return dateLib.format(date, "d");
 }
diff --git a/src/formatters/formatMonthDropdown.ts b/src/formatters/formatMonthDropdown.ts
index b7773faca..02be49a9d 100644
--- a/src/formatters/formatMonthDropdown.ts
+++ b/src/formatters/formatMonthDropdown.ts
@@ -1,5 +1,5 @@
 import type { DateFnsMonth } from "../lib/dateLib.js";
-import { enUS } from "../lib/locales.js";
+import { defaultLocale } from "../lib/locales.js";
 
 /**
  * Format the month number for the dropdown option label.
@@ -12,7 +12,7 @@ export function formatMonthDropdown(
   /** The month number to format. */
   monthNumber: number,
   /** The locale to use for formatting. */
-  locale = enUS
+  locale = defaultLocale
 ): string {
   return locale.localize?.month(monthNumber as DateFnsMonth);
 }
diff --git a/src/formatters/formatWeekdayName.ts b/src/formatters/formatWeekdayName.ts
index f29fc140d..6685800be 100644
--- a/src/formatters/formatWeekdayName.ts
+++ b/src/formatters/formatWeekdayName.ts
@@ -1,5 +1,4 @@
-import type { FormatOptions, DateLib } from "../lib/dateLib.js";
-import { dateLib as defaultDateLib } from "../lib/index.js";
+import { DateLib, type FormatOptions } from "../lib/dateLib.js";
 
 /**
  * Format the weekday name to be displayed in the weekdays header.
@@ -12,7 +11,7 @@ export function formatWeekdayName(
   weekday: Date,
   options?: FormatOptions,
   /** @ignore */
-  dateLib: DateLib = defaultDateLib
+  dateLib: DateLib = DateLib.fromOptionsDefaultLocale(options)
 ) {
-  return dateLib.format(weekday, "cccccc", options);
+  return dateLib.format(weekday, "cccccc");
 }
diff --git a/src/helpers/getDateLib.ts b/src/helpers/getDateLib.ts
deleted file mode 100644
index 2251d8d87..000000000
--- a/src/helpers/getDateLib.ts
+++ /dev/null
@@ -1,9 +0,0 @@
-import { dateLib, type DateLib } from "../lib/dateLib.js";
-import type { DayPickerProps } from "../types/index.js";
-
-export function getDateLib(customLib: DayPickerProps["dateLib"]): DateLib {
-  return {
-    ...dateLib,
-    ...customLib
-  };
-}
diff --git a/src/helpers/getDates.test.ts b/src/helpers/getDates.test.ts
index 40cd1a05b..05fdaabef 100644
--- a/src/helpers/getDates.test.ts
+++ b/src/helpers/getDates.test.ts
@@ -1,4 +1,4 @@
-import { dateLib } from "../lib";
+import { DateLib } from "../lib/dateLib";
 
 import { getDates } from "./getDates";
 
@@ -13,7 +13,7 @@ describe("when the first month and the last month are the same", () => {
           {
             fixedWeeks: false
           },
-          dateLib
+          DateLib.fromOptionsDefaultLocale({})
         );
         expect(dates).toHaveLength(42);
         expect(dates[0]).toEqual(new Date(2023, 10, 26));
@@ -28,7 +28,7 @@ describe("when the first month and the last month are the same", () => {
           {
             fixedWeeks: true
           },
-          dateLib
+          DateLib.fromOptionsDefaultLocale({})
         );
         expect(dates).toHaveLength(42);
         expect(dates[0]).toEqual(new Date(2023, 10, 26));
@@ -46,7 +46,7 @@ describe("when the first month and the last month are the same", () => {
           {
             fixedWeeks: false
           },
-          dateLib
+          DateLib.fromOptionsDefaultLocale({})
         );
         expect(dates).toHaveLength(35);
         expect(dates[0]).toEqual(new Date(2023, 3, 30));
@@ -61,7 +61,7 @@ describe("when the first month and the last month are the same", () => {
           {
             fixedWeeks: true
           },
-          dateLib
+          DateLib.fromOptionsDefaultLocale({})
         );
         expect(dates).toHaveLength(42);
         expect(dates[0]).toEqual(new Date(2023, 3, 30));
@@ -75,10 +75,8 @@ describe("when the first month and the last month are the same", () => {
       const dates = getDates(
         [month],
         undefined,
-        {
-          weekStartsOn: 1
-        },
-        dateLib
+        {},
+        DateLib.fromOptionsDefaultLocale({ weekStartsOn: 1 })
       );
       expect(dates[0]).toBeMonday();
       expect(dates[0]).toEqual(new Date(2023, 4, 1));
@@ -90,7 +88,12 @@ describe("when the first month and the last month are the same", () => {
     const maxDate = new Date(2023, 4, 15);
 
     it("the last day should be the max date", () => {
-      const dates = getDates([month], maxDate, { weekStartsOn: 1 }, dateLib);
+      const dates = getDates(
+        [month],
+        maxDate,
+        {},
+        DateLib.fromOptionsDefaultLocale({ weekStartsOn: 1 })
+      );
       expect(dates).toHaveLength(15);
       expect(dates[dates.length - 1]).toEqual(maxDate);
     });
@@ -98,7 +101,12 @@ describe("when the first month and the last month are the same", () => {
   describe("when using ISO weeks", () => {
     const month = new Date(2023, 4, 1);
     it("the first day should be Monday", () => {
-      const dates = getDates([month], undefined, { ISOWeek: true }, dateLib);
+      const dates = getDates(
+        [month],
+        undefined,
+        { ISOWeek: true },
+        DateLib.fromOptionsDefaultLocale({})
+      );
       expect(dates[0]).toBeMonday();
       expect(dates[0]).toEqual(new Date(2023, 4, 1));
       expect(dates[dates.length - 1]).toEqual(new Date(2023, 5, 4));
@@ -114,10 +122,8 @@ describe("when the first month and the last month are different", () => {
       const dates = getDates(
         [firstMonth, lastMonth],
         undefined,
-        {
-          fixedWeeks: false
-        },
-        dateLib
+        { fixedWeeks: false },
+        DateLib.fromOptionsDefaultLocale({})
       );
       expect(dates).toHaveLength(252);
       expect(dates[0]).toEqual(new Date(2023, 3, 30));
@@ -133,10 +139,8 @@ describe("when the first month and the last month are different", () => {
       const dates = getDates(
         [firstMonth, lastMonth],
         maxDate,
-        {
-          weekStartsOn: 1
-        },
-        dateLib
+        {},
+        DateLib.fromOptionsDefaultLocale({ weekStartsOn: 1 })
       );
       expect(dates).toHaveLength(46);
       expect(dates[dates.length - 1]).toEqual(maxDate);
@@ -145,7 +149,12 @@ describe("when the first month and the last month are different", () => {
   describe("when using ISO weeks", () => {
     const month = new Date(2023, 4, 1);
     it("the first day should be Monday", () => {
-      const dates = getDates([month], undefined, { ISOWeek: true }, dateLib);
+      const dates = getDates(
+        [month],
+        undefined,
+        { ISOWeek: true },
+        DateLib.fromOptionsDefaultLocale({})
+      );
       expect(dates[0]).toBeMonday();
       expect(dates[0]).toEqual(new Date(2023, 4, 1));
       expect(dates[dates.length - 1]).toEqual(new Date(2023, 5, 4));
diff --git a/src/helpers/getDates.ts b/src/helpers/getDates.ts
index 2cf7e7209..533597c1e 100644
--- a/src/helpers/getDates.ts
+++ b/src/helpers/getDates.ts
@@ -7,16 +7,13 @@ const NrOfDaysWithFixedWeeks = 42;
 export function getDates(
   displayMonths: Date[],
   maxDate: Date | undefined,
-  props: Pick<
-    DayPickerProps,
-    "ISOWeek" | "fixedWeeks" | "locale" | "weekStartsOn" | "timeZone"
-  >,
+  props: Pick<DayPickerProps, "ISOWeek" | "fixedWeeks">,
   dateLib: DateLib
 ): Date[] {
   const firstMonth = displayMonths[0];
   const lastMonth = displayMonths[displayMonths.length - 1];
 
-  const { ISOWeek, fixedWeeks, locale, weekStartsOn } = props ?? {};
+  const { ISOWeek, fixedWeeks } = props ?? {};
   const {
     startOfWeek,
     endOfWeek,
@@ -31,17 +28,11 @@ export function getDates(
 
   const startWeekFirstDate = ISOWeek
     ? startOfISOWeek(firstMonth)
-    : startOfWeek(firstMonth, {
-        weekStartsOn,
-        locale
-      });
+    : startOfWeek(firstMonth);
 
   const endWeekLastDate = ISOWeek
     ? endOfISOWeek(endOfMonth(lastMonth))
-    : endOfWeek(endOfMonth(lastMonth), {
-        weekStartsOn,
-        locale
-      });
+    : endOfWeek(endOfMonth(lastMonth));
 
   const nOfDays = differenceInCalendarDays(endWeekLastDate, startWeekFirstDate);
   const nOfMonths = differenceInCalendarMonths(lastMonth, firstMonth) + 1;
diff --git a/src/helpers/getDisplayMonths.test.ts b/src/helpers/getDisplayMonths.test.ts
index 320851d3e..156722ba6 100644
--- a/src/helpers/getDisplayMonths.test.ts
+++ b/src/helpers/getDisplayMonths.test.ts
@@ -1,4 +1,4 @@
-import { dateLib } from "react-day-picker";
+import { DateLib } from "../lib/dateLib";
 
 import { getDisplayMonths } from "./getDisplayMonths";
 
@@ -6,7 +6,7 @@ describe("getDisplayMonths", () => {
   it("should return the months to display in the calendar", () => {
     const firstMonth = new Date(2020, 0);
     const expectedResult = [new Date(2020, 0)];
-    const result = getDisplayMonths(firstMonth, undefined, {}, dateLib);
+    const result = getDisplayMonths(firstMonth, undefined, {}, new DateLib());
     expect(result).toEqual(expectedResult);
   });
 
@@ -23,7 +23,7 @@ describe("getDisplayMonths", () => {
       {
         numberOfMonths: 3
       },
-      dateLib
+      new DateLib()
     );
     expect(result).toEqual(expectedResult);
   });
@@ -37,7 +37,7 @@ describe("getDisplayMonths", () => {
       {
         numberOfMonths: 3
       },
-      dateLib
+      new DateLib()
     );
     expect(result).toEqual(expectedResult);
   });
diff --git a/src/helpers/getFocusableDate.ts b/src/helpers/getFocusableDate.ts
index 1e1b03475..32389a3fa 100644
--- a/src/helpers/getFocusableDate.ts
+++ b/src/helpers/getFocusableDate.ts
@@ -12,10 +12,10 @@ export function getFocusableDate(
   refDate: Date,
   navStart: Date | undefined,
   navEnd: Date | undefined,
-  props: Pick<DayPickerProps, "locale" | "ISOWeek" | "weekStartsOn">,
+  props: Pick<DayPickerProps, "ISOWeek">,
   dateLib: DateLib
 ): Date {
-  const { weekStartsOn, locale, ISOWeek } = props;
+  const { ISOWeek } = props;
   const {
     addDays,
     addMonths,
@@ -34,11 +34,8 @@ export function getFocusableDate(
     month: addMonths,
     year: addYears,
     startOfWeek: (date: Date) =>
-      ISOWeek
-        ? startOfISOWeek(date)
-        : startOfWeek(date, { locale, weekStartsOn }),
-    endOfWeek: (date: Date) =>
-      ISOWeek ? endOfISOWeek(date) : endOfWeek(date, { locale, weekStartsOn })
+      ISOWeek ? startOfISOWeek(date) : startOfWeek(date),
+    endOfWeek: (date: Date) => (ISOWeek ? endOfISOWeek(date) : endOfWeek(date))
   };
 
   let focusableDate = moveFns[moveBy](refDate, moveDir === "after" ? 1 : -1);
diff --git a/src/helpers/getFormatters.test.ts b/src/helpers/getFormatters.test.ts
index 7fb7baf8c..9f17bbbac 100644
--- a/src/helpers/getFormatters.test.ts
+++ b/src/helpers/getFormatters.test.ts
@@ -1,5 +1,5 @@
 import * as defaultFormatters from "../formatters";
-import { dateLib } from "../lib";
+import { DateLib } from "../lib/dateLib";
 
 import { getFormatters } from "./getFormatters";
 
@@ -20,8 +20,10 @@ test("merges custom formatters with default formatters", () => {
 
 test("assigns `formatMonthCaption` to `formatCaption` if `formatCaption` is not defined", () => {
   const result = getFormatters({ formatMonthCaption: () => "customMonth" });
-  expect(result.formatCaption(new Date(), {}, dateLib)).toBe("customMonth");
-  expect(result.formatMonthCaption(new Date(), {}, dateLib)).toBe(
+  expect(result.formatCaption(new Date(), {}, new DateLib())).toBe(
+    "customMonth"
+  );
+  expect(result.formatMonthCaption(new Date(), {}, new DateLib())).toBe(
     "customMonth"
   );
 });
@@ -31,8 +33,10 @@ test("does not overwrite `formatCaption` if already defined", () => {
     formatMonthCaption: () => "customMonth",
     formatCaption: () => "customCaption"
   });
-  expect(result.formatCaption(new Date(), {}, dateLib)).toBe("customCaption");
-  expect(result.formatMonthCaption(new Date(), {}, dateLib)).toBe(
+  expect(result.formatCaption(new Date(), {}, new DateLib())).toBe(
+    "customCaption"
+  );
+  expect(result.formatMonthCaption(new Date(), {}, new DateLib())).toBe(
     "customMonth"
   );
 });
diff --git a/src/helpers/getInitialMonth.test.ts b/src/helpers/getInitialMonth.test.ts
index e208102ab..198607e12 100644
--- a/src/helpers/getInitialMonth.test.ts
+++ b/src/helpers/getInitialMonth.test.ts
@@ -1,5 +1,6 @@
 import { addMonths, isSameMonth } from "date-fns";
-import { dateLib } from "react-day-picker";
+
+import { DateLib } from "../lib/dateLib";
 
 import { getInitialMonth } from "./getInitialMonth";
 
@@ -7,21 +8,21 @@ describe("when no endMonth is given", () => {
   describe("when month is in context", () => {
     const month = new Date(2010, 11, 12);
     it("return that month", () => {
-      const startMonth = getInitialMonth({ month }, dateLib);
+      const startMonth = getInitialMonth({ month }, new DateLib());
       expect(isSameMonth(startMonth, month)).toBe(true);
     });
   });
   describe("when defaultMonth is in context", () => {
     const defaultMonth = new Date(2010, 11, 12);
     it("return that month", () => {
-      const startMonth = getInitialMonth({ defaultMonth }, dateLib);
+      const startMonth = getInitialMonth({ defaultMonth }, new DateLib());
       expect(isSameMonth(startMonth, defaultMonth)).toBe(true);
     });
   });
   describe("when no month or defaultMonth", () => {
     const today = new Date(2010, 11, 12);
     it("return the today month", () => {
-      const startMonth = getInitialMonth({ today }, dateLib);
+      const startMonth = getInitialMonth({ today }, new DateLib());
       expect(isSameMonth(startMonth, today)).toBe(true);
     });
   });
@@ -32,7 +33,7 @@ describe("when endMonth is given", () => {
     const endMonth = addMonths(month, -2);
     describe("when the number of month is 1", () => {
       it("return the endMonth", () => {
-        const startMonth = getInitialMonth({ month, endMonth }, dateLib);
+        const startMonth = getInitialMonth({ month, endMonth }, new DateLib());
         expect(isSameMonth(startMonth, endMonth)).toBe(true);
       });
     });
@@ -40,7 +41,7 @@ describe("when endMonth is given", () => {
       it("return the endMonth plus the number of months", () => {
         const startMonth = getInitialMonth(
           { month, numberOfMonths: 3, endMonth },
-          dateLib
+          new DateLib()
         );
         const expectedMonth = addMonths(endMonth, -1 * (3 - 1));
         expect(isSameMonth(startMonth, expectedMonth)).toBe(true);
diff --git a/src/helpers/getMonthOptions.test.ts b/src/helpers/getMonthOptions.test.ts
index a5ecc771a..6fb71862d 100644
--- a/src/helpers/getMonthOptions.test.ts
+++ b/src/helpers/getMonthOptions.test.ts
@@ -1,6 +1,7 @@
 import { type Locale, format } from "date-fns";
 import { enUS as locale } from "date-fns/locale";
-import { dateLib } from "react-day-picker";
+
+import { DateLib } from "../lib/dateLib";
 
 import { getFormatters } from "./getFormatters";
 import { getMonthOptions } from "./getMonthOptions";
@@ -18,8 +19,7 @@ test("return correct dropdown options", () => {
     startMonth,
     endMonth,
     formatters,
-    locale,
-    dateLib
+    new DateLib({ locale })
   );
 
   expect(result).toEqual([
diff --git a/src/helpers/getMonthOptions.ts b/src/helpers/getMonthOptions.ts
index 86b951692..4d890e12d 100644
--- a/src/helpers/getMonthOptions.ts
+++ b/src/helpers/getMonthOptions.ts
@@ -1,5 +1,5 @@
 import { DropdownOption } from "../components/Dropdown.js";
-import type { Locale, DateLib } from "../lib/dateLib.js";
+import type { DateLib } from "../lib/dateLib.js";
 import type { Formatters } from "../types/index.js";
 
 /** Return the months to show in the dropdown. */
@@ -8,7 +8,6 @@ export function getMonthOptions(
   navStart: Date | undefined,
   navEnd: Date | undefined,
   formatters: Pick<Formatters, "formatMonthDropdown">,
-  locale: Locale | undefined,
   dateLib: DateLib
 ): DropdownOption[] | undefined {
   if (!navStart) return undefined;
@@ -27,7 +26,7 @@ export function getMonthOptions(
     return a - b;
   });
   const options = sortedMonths.map((value) => {
-    const label = formatters.formatMonthDropdown(value, locale);
+    const label = formatters.formatMonthDropdown(value, dateLib.locale);
     const month = dateLib.Date
       ? new dateLib.Date(year, value)
       : new Date(year, value);
diff --git a/src/helpers/getMonths.test.ts b/src/helpers/getMonths.test.ts
index 482fbde29..e1208b4f6 100644
--- a/src/helpers/getMonths.test.ts
+++ b/src/helpers/getMonths.test.ts
@@ -1,7 +1,7 @@
 import { DayPickerProps } from "react-day-picker";
 
 import { CalendarMonth } from "../classes";
-import { dateLib } from "../lib";
+import { DateLib } from "../lib/dateLib";
 
 import { getMonths } from "./getMonths";
 
@@ -17,20 +17,17 @@ const mockDates = [
 
 const mockProps: Pick<
   DayPickerProps,
-  | "fixedWeeks"
-  | "ISOWeek"
-  | "locale"
-  | "weekStartsOn"
-  | "reverseMonths"
-  | "firstWeekContainsDate"
+  "fixedWeeks" | "ISOWeek" | "reverseMonths"
 > = {
   fixedWeeks: false,
   ISOWeek: false,
-  locale: undefined,
+  reverseMonths: false
+};
+
+const dateLib = DateLib.fromOptionsDefaultLocale({
   weekStartsOn: 0, // Sunday
-  reverseMonths: false,
   firstWeekContainsDate: 1
-};
+});
 
 it("should return the correct months without ISO weeks and reverse months", () => {
   const displayMonths = [new Date(2023, 5, 1)]; // June 2023
diff --git a/src/helpers/getMonths.ts b/src/helpers/getMonths.ts
index fad0d29f8..147979529 100644
--- a/src/helpers/getMonths.ts
+++ b/src/helpers/getMonths.ts
@@ -9,16 +9,7 @@ export function getMonths(
   /** The dates to display in the calendar. */
   dates: Date[],
   /** Options from the props context. */
-  props: Pick<
-    DayPickerProps,
-    | "fixedWeeks"
-    | "ISOWeek"
-    | "locale"
-    | "weekStartsOn"
-    | "reverseMonths"
-    | "firstWeekContainsDate"
-    | "timeZone"
-  >,
+  props: Pick<DayPickerProps, "fixedWeeks" | "ISOWeek" | "reverseMonths">,
   dateLib: DateLib
 ): CalendarMonth[] {
   const {
@@ -35,17 +26,11 @@ export function getMonths(
     (months, month) => {
       const firstDateOfFirstWeek = props.ISOWeek
         ? startOfISOWeek(month)
-        : startOfWeek(month, {
-            locale: props.locale,
-            weekStartsOn: props.weekStartsOn
-          });
+        : startOfWeek(month);
 
       const lastDateOfLastWeek = props.ISOWeek
         ? endOfISOWeek(endOfMonth(month))
-        : endOfWeek(endOfMonth(month), {
-            locale: props.locale,
-            weekStartsOn: props.weekStartsOn
-          });
+        : endOfWeek(endOfMonth(month));
 
       /** The dates to display in the month. */
       const monthDates = dates.filter((date) => {
@@ -63,13 +48,7 @@ export function getMonths(
 
       const weeks: CalendarWeek[] = monthDates.reduce<CalendarWeek[]>(
         (weeks, date) => {
-          const weekNumber = props.ISOWeek
-            ? getISOWeek(date)
-            : getWeek(date, {
-                locale: props.locale,
-                weekStartsOn: props.weekStartsOn,
-                firstWeekContainsDate: props.firstWeekContainsDate
-              });
+          const weekNumber = props.ISOWeek ? getISOWeek(date) : getWeek(date);
           const week = weeks.find((week) => week.weekNumber === weekNumber);
 
           const day = new CalendarDay(date, month, dateLib);
diff --git a/src/helpers/getNavMonth.test.ts b/src/helpers/getNavMonth.test.ts
index 850dcfa20..bd4c9f6da 100644
--- a/src/helpers/getNavMonth.test.ts
+++ b/src/helpers/getNavMonth.test.ts
@@ -1,10 +1,10 @@
-import { dateLib } from "../lib";
+import { DateLib } from "../lib/dateLib";
 
 import { getNavMonths } from "./getNavMonth";
 
 describe('when "startMonth" is not passed in', () => {
   test('"startMonth" should be undefined', () => {
-    const [navStartMonth] = getNavMonths({}, dateLib);
+    const [navStartMonth] = getNavMonths({}, new DateLib());
     expect(navStartMonth).toBeUndefined();
   });
 });
@@ -13,7 +13,7 @@ describe('when "startMonth" is passed in', () => {
     {
       startMonth: new Date(2021, 4, 3)
     },
-    dateLib
+    new DateLib()
   );
   test('"startMonth" should be the start of that month', () => {
     expect(navStartMonth).toEqual(new Date(2021, 4, 1));
@@ -25,7 +25,7 @@ describe('when "startMonth" is passed in', () => {
   });
 });
 describe('when "fromYear" is passed in', () => {
-  const [navStartMonth] = getNavMonths({ fromYear: 2021 }, dateLib);
+  const [navStartMonth] = getNavMonths({ fromYear: 2021 }, new DateLib());
   test('"startMonth" should be the start of that year', () => {
     expect(navStartMonth).toEqual(new Date(2021, 0, 1));
   });
@@ -35,7 +35,7 @@ describe('when "endMonth" is passed in', () => {
     {
       endMonth: new Date(2021, 4, 3)
     },
-    dateLib
+    new DateLib()
   );
   test('"endMonth" should be the end of that month', () => {
     expect(navEndMonth).toEqual(new Date(2021, 4, 31));
@@ -50,7 +50,7 @@ describe('when "endMonth" is passed in', () => {
 describe('when "toYear" is passed in', () => {
   const toYear = 2021;
   const expectedendMonth = new Date(2021, 11, 31);
-  const [, navEndMonth] = getNavMonths({ toYear }, dateLib);
+  const [, navEndMonth] = getNavMonths({ toYear }, new DateLib());
   test('"endMonth" should be the end of that year', () => {
     expect(navEndMonth).toEqual(expectedendMonth);
   });
@@ -63,7 +63,7 @@ describe('when "captionLayout" is dropdown', () => {
       captionLayout: "dropdown",
       today
     },
-    dateLib
+    new DateLib()
   );
   test('"startMonth" should be 100 years ago', () => {
     expect(navStartMonth).toEqual(new Date(1924, 0, 1));
@@ -80,7 +80,7 @@ describe('when "captionLayout" is dropdown', () => {
         fromYear,
         today
       },
-      dateLib
+      new DateLib()
     );
     test('"startMonth" should be equal to the "fromYear"', () => {
       expect(navStartMonth).toEqual(new Date(2022, 0, 1));
@@ -98,7 +98,7 @@ describe('when "captionLayout" is dropdown', () => {
         toYear,
         today
       },
-      dateLib
+      new DateLib()
     );
     test('"startMonth" should be 100 years ago', () => {
       expect(navStartMonth).toEqual(new Date(1921, 0, 1));
diff --git a/src/helpers/getNavMonth.ts b/src/helpers/getNavMonth.ts
index 69db4e2e3..d8e221a6d 100644
--- a/src/helpers/getNavMonth.ts
+++ b/src/helpers/getNavMonth.ts
@@ -12,7 +12,6 @@ export function getNavMonths(
     | "startMonth"
     | "today"
     | "timeZone"
-    | "dateLib"
     // Deprecated:
     | "fromMonth"
     | "fromYear"
diff --git a/src/helpers/getNextFocus.test.tsx b/src/helpers/getNextFocus.test.tsx
index 364133a3e..48407c576 100644
--- a/src/helpers/getNextFocus.test.tsx
+++ b/src/helpers/getNextFocus.test.tsx
@@ -1,23 +1,22 @@
 import { CalendarDay } from "../classes";
-import { dateLib } from "../lib";
+import { DateLib } from "../lib/dateLib";
 import type { DayPickerProps, MoveFocusBy, MoveFocusDir } from "../types";
 
 import { getNextFocus } from "./getNextFocus";
 
 const props: Pick<
   DayPickerProps,
-  "disabled" | "hidden" | "startMonth" | "endMonth" | "dateLib"
+  "disabled" | "hidden" | "startMonth" | "endMonth"
 > = {
   disabled: [],
-  hidden: [],
-  dateLib
+  hidden: []
 };
 
 it("should return `undefined` if `attempt` exceeds 365", () => {
   const focusedDay = new CalendarDay(
     new Date(2020, 0, 1),
     new Date(2020, 0, 1),
-    dateLib
+    new DateLib()
   );
   const moveBy: MoveFocusBy = "day";
   const moveDir: MoveFocusDir = "after";
@@ -28,7 +27,7 @@ it("should return `undefined` if `attempt` exceeds 365", () => {
     undefined,
     undefined,
     props,
-    dateLib,
+    new DateLib(),
     366
   );
   expect(result).toBeUndefined();
@@ -38,7 +37,7 @@ it("should return the focus date if it is not disabled or hidden", () => {
   const focusedDay = new CalendarDay(
     new Date(2020, 0, 1),
     new Date(2020, 0, 1),
-    dateLib
+    new DateLib()
   );
   const expectedDate = new Date(2020, 0, 2);
   const result = getNextFocus(
@@ -48,7 +47,7 @@ it("should return the focus date if it is not disabled or hidden", () => {
     undefined,
     undefined,
     props,
-    dateLib
+    new DateLib()
   );
   expect(result?.date).toEqual(expectedDate);
 });
@@ -57,7 +56,7 @@ it("should return the next focus date if it is disabled", () => {
   const focusedDay = new CalendarDay(
     new Date(2020, 0, 1),
     new Date(2020, 0, 1),
-    dateLib
+    new DateLib()
   );
   const disabledDate = new Date(2020, 0, 2);
   const expectedDate = new Date(2020, 0, 3);
@@ -71,7 +70,7 @@ it("should return the next focus date if it is disabled", () => {
       ...props,
       disabled: [disabledDate]
     },
-    dateLib
+    new DateLib()
   );
   expect(result?.date).toEqual(expectedDate);
 });
@@ -80,7 +79,7 @@ it("should return the next focus date if it is hidden", () => {
   const focusedDay = new CalendarDay(
     new Date(2020, 0, 1),
     new Date(2020, 0, 1),
-    dateLib
+    new DateLib()
   );
   const hiddenDate = new Date(2020, 0, 2);
   const expectedDate = new Date(2020, 0, 3);
@@ -94,7 +93,7 @@ it("should return the next focus date if it is hidden", () => {
       ...props,
       hidden: [hiddenDate]
     },
-    dateLib
+    new DateLib()
   );
   expect(result?.date).toEqual(expectedDate);
 });
diff --git a/src/helpers/getNextFocus.tsx b/src/helpers/getNextFocus.tsx
index 06591e964..eef5d9ba1 100644
--- a/src/helpers/getNextFocus.tsx
+++ b/src/helpers/getNextFocus.tsx
@@ -18,13 +18,7 @@ export function getNextFocus(
   calendarEndMonth: Date | undefined,
   props: Pick<
     DayPickerProps,
-    | "disabled"
-    | "hidden"
-    | "modifiers"
-    | "locale"
-    | "ISOWeek"
-    | "weekStartsOn"
-    | "timeZone"
+    "disabled" | "hidden" | "modifiers" | "ISOWeek" | "timeZone"
   >,
   dateLib: DateLib,
   attempt: number = 0
diff --git a/src/helpers/getNextMonth.test.ts b/src/helpers/getNextMonth.test.ts
index 616b1a3f1..78f7effe2 100644
--- a/src/helpers/getNextMonth.test.ts
+++ b/src/helpers/getNextMonth.test.ts
@@ -1,5 +1,6 @@
 import { addMonths, isSameMonth } from "date-fns";
-import { dateLib } from "react-day-picker";
+
+import { DateLib } from "../lib/dateLib";
 
 import { getNextMonth } from "./getNextMonth";
 
@@ -14,7 +15,7 @@ describe("when number of months is 1", () => {
         {
           disableNavigation: true
         },
-        dateLib
+        new DateLib()
       );
       expect(result).toBe(undefined);
     });
@@ -22,7 +23,7 @@ describe("when number of months is 1", () => {
   describe("when in the navigable range", () => {
     const endMonth = addMonths(startingMonth, 3);
     it("the next month is not undefined", () => {
-      const result = getNextMonth(startingMonth, endMonth, {}, dateLib);
+      const result = getNextMonth(startingMonth, endMonth, {}, new DateLib());
       const expectedNextMonth = addMonths(startingMonth, 1);
       expect(result && isSameMonth(result, expectedNextMonth)).toBeTruthy();
     });
@@ -30,7 +31,7 @@ describe("when number of months is 1", () => {
   describe("when not in the navigable range", () => {
     const endMonth = startingMonth;
     it("the next month is undefined", () => {
-      const result = getNextMonth(startingMonth, endMonth, {}, dateLib);
+      const result = getNextMonth(startingMonth, endMonth, {}, new DateLib());
       expect(result).toBe(undefined);
     });
   });
@@ -47,7 +48,7 @@ describe("when displaying 3 months", () => {
           numberOfMonths,
           pagedNavigation
         },
-        dateLib
+        new DateLib()
       );
       const expectedNextMonth = addMonths(startingMonth, 3);
       expect(result && isSameMonth(result, expectedNextMonth)).toBeTruthy();
@@ -61,7 +62,7 @@ describe("when displaying 3 months", () => {
             numberOfMonths,
             pagedNavigation
           },
-          dateLib
+          new DateLib()
         );
         expect(result).toBe(undefined);
       });
@@ -77,7 +78,7 @@ describe("when displaying 3 months", () => {
           numberOfMonths,
           pagedNavigation
         },
-        dateLib
+        new DateLib()
       );
       const expectedNextMonth = addMonths(startingMonth, 1);
       expect(result && isSameMonth(result, expectedNextMonth)).toBeTruthy();
@@ -91,7 +92,7 @@ describe("when displaying 3 months", () => {
             numberOfMonths,
             pagedNavigation
           },
-          dateLib
+          new DateLib()
         );
         expect(result).toBe(undefined);
       });
diff --git a/src/helpers/getPossibleFocusDate.test.ts b/src/helpers/getPossibleFocusDate.test.ts
index 9d00eb88e..b3f66fb83 100644
--- a/src/helpers/getPossibleFocusDate.test.ts
+++ b/src/helpers/getPossibleFocusDate.test.ts
@@ -11,17 +11,18 @@ import {
   endOfWeek
 } from "date-fns";
 
+import { DateLib } from "../lib/dateLib";
 import type { DayPickerProps, MoveFocusBy, MoveFocusDir } from "../types";
 
-import { dateLib } from "..";
 import { getFocusableDate } from "./getFocusableDate";
 
 const focusedDate = new Date(2023, 0, 1); // Jan 1, 2023
-const options: Pick<DayPickerProps, "locale" | "ISOWeek" | "weekStartsOn"> = {
-  locale: undefined,
-  ISOWeek: false,
-  weekStartsOn: 0 // Sunday
+const options: Pick<DayPickerProps, "ISOWeek"> = {
+  ISOWeek: false
 };
+const dateLib = DateLib.fromOptionsDefaultLocale({
+  weekStartsOn: 0 // Sunday
+});
 
 const calendarStartMonth = new Date(2022, 0, 1); // Jan 1, 2022
 const calendarEndMonth = new Date(2024, 0, 1); // Jan 1, 2024
diff --git a/src/helpers/getPreviousMonth.test.ts b/src/helpers/getPreviousMonth.test.ts
index eecca21f7..e6bf52541 100644
--- a/src/helpers/getPreviousMonth.test.ts
+++ b/src/helpers/getPreviousMonth.test.ts
@@ -1,4 +1,5 @@
-import { dateLib } from "..";
+import { DateLib } from "../lib/dateLib";
+
 import { getPreviousMonth } from "./getPreviousMonth";
 
 it("should return undefined if navigation is disabled", () => {
@@ -14,7 +15,7 @@ it("should return undefined if navigation is disabled", () => {
     firstDisplayedMonth,
     calendarStartMonth,
     props,
-    dateLib
+    new DateLib()
   );
 
   expect(result).toBeUndefined();
@@ -32,7 +33,7 @@ it("should return the previous month if startMonth is not provided", () => {
     firstDisplayedMonth,
     undefined,
     props,
-    dateLib
+    new DateLib()
   );
 
   expect(result).toEqual(new Date(2022, 0, 1)); // January 2022
@@ -50,7 +51,7 @@ it("should return undefined if the previous month is before the startMonth", ()
     firstDisplayedMonth,
     calendarStartMonth,
     props,
-    dateLib
+    new DateLib()
   );
   expect(result).toBeUndefined();
 });
@@ -62,15 +63,14 @@ it("should return the correct previous month when pagedNavigation is true", () =
     disableNavigation: false,
     pagedNavigation: true,
     numberOfMonths: 2,
-    startMonth: new Date(2022, 0, 1),
-    dateLib
+    startMonth: new Date(2022, 0, 1)
   };
 
   const result = getPreviousMonth(
     firstDisplayedMonth,
     calendarStartMonth,
     props,
-    dateLib
+    new DateLib()
   );
 
   expect(result).toEqual(new Date(2022, 0, 1)); // January 2022
diff --git a/src/helpers/getWeekdays.test.ts b/src/helpers/getWeekdays.test.ts
index 711da7c4c..0c3935460 100644
--- a/src/helpers/getWeekdays.test.ts
+++ b/src/helpers/getWeekdays.test.ts
@@ -1,6 +1,6 @@
 import { es } from "date-fns/locale/es";
 
-import { dateLib } from "../lib";
+import { DateLib } from "../lib/dateLib";
 
 import { getWeekdays } from "./getWeekdays";
 
@@ -8,7 +8,7 @@ let result: Date[];
 
 describe("when rendered without a locale", () => {
   beforeEach(() => {
-    result = getWeekdays();
+    result = getWeekdays(DateLib.fromOptionsDefaultLocale({}));
   });
   test("should return 7 days", () => {
     expect(result).toHaveLength(7);
@@ -22,7 +22,9 @@ describe.each<0 | 1 | 2 | 3 | 4 | 5 | 6>([0, 1, 2, 3, 4, 5, 6])(
   "when week start on %s",
   (weekStartsOn) => {
     beforeEach(() => {
-      result = getWeekdays(es, weekStartsOn);
+      result = getWeekdays(
+        DateLib.fromOptionsDefaultLocale({ locale: es, weekStartsOn })
+      );
     });
     test("the first date should be weekStartsOn", () => {
       expect(result[0].getDay()).toBe(weekStartsOn);
@@ -32,7 +34,11 @@ describe.each<0 | 1 | 2 | 3 | 4 | 5 | 6>([0, 1, 2, 3, 4, 5, 6])(
 
 describe("when using ISO week", () => {
   beforeEach(() => {
-    result = getWeekdays(es, 3, true, undefined, dateLib);
+    result = getWeekdays(
+      DateLib.fromOptionsDefaultLocale({ locale: es, weekStartsOn: 3 }),
+      true,
+      undefined
+    );
   });
   test("should return Monday as first day", () => {
     expect(result[0]).toBeMonday();
diff --git a/src/helpers/getWeekdays.ts b/src/helpers/getWeekdays.ts
index 57552ebbf..5b1a7dc38 100644
--- a/src/helpers/getWeekdays.ts
+++ b/src/helpers/getWeekdays.ts
@@ -1,21 +1,17 @@
 import { TZDate } from "@date-fns/tz";
 
-import type { Locale, DateLib } from "../lib/dateLib.js";
-import { dateLib as defaultDateLib } from "../lib/index.js";
+import { DateLib } from "../lib/dateLib.js";
 
 /**
  * Generate a series of 7 days, starting from the week, to use for formatting
  * the weekday names (Monday, Tuesday, etc.).
  */
 export function getWeekdays(
-  locale?: Locale | undefined,
-  /** The index of the first day of the week (0 - Sunday). */
-  weekStartsOn?: 0 | 1 | 2 | 3 | 4 | 5 | 6 | undefined,
+  /** The date library. */
+  dateLib: DateLib,
   /** Use ISOWeek instead of locale/ */
   ISOWeek?: boolean | undefined,
-  timeZone?: string | undefined,
-  /** @ignore */
-  dateLib: DateLib = defaultDateLib
+  timeZone?: string | undefined
 ): Date[] {
   const date = timeZone
     ? TZDate.tz(timeZone)
@@ -24,7 +20,7 @@ export function getWeekdays(
       : new Date();
   const start = ISOWeek
     ? dateLib.startOfISOWeek(date)
-    : dateLib.startOfWeek(date, { locale, weekStartsOn });
+    : dateLib.startOfWeek(date);
 
   const days = [];
   for (let i = 0; i < 7; i++) {
diff --git a/src/helpers/getYearOptions.test.ts b/src/helpers/getYearOptions.test.ts
index 79d2879a3..a2e3b2c38 100644
--- a/src/helpers/getYearOptions.test.ts
+++ b/src/helpers/getYearOptions.test.ts
@@ -1,4 +1,4 @@
-import { dateLib } from "react-day-picker";
+import { DateLib } from "../lib/dateLib";
 
 import { getFormatters } from "./getFormatters";
 import { getYearOptions } from "./getYearOptions";
@@ -13,14 +13,14 @@ test("return undefined if startMonth or endMonth is not provided", () => {
     undefined,
     new Date(2022, 11, 31),
     formatters,
-    dateLib
+    new DateLib()
   );
   const result2 = getYearOptions(
     displayMonth,
     new Date(2022, 0, 1),
     undefined,
     formatters,
-    dateLib
+    new DateLib()
   );
 
   expect(result1).toBeUndefined();
@@ -40,7 +40,7 @@ test("return correct dropdown options", () => {
     startMonth,
     endMonth,
     formatters,
-    dateLib
+    new DateLib()
   );
 
   expect(result).toEqual([
diff --git a/src/index.ts b/src/index.ts
index 35598d546..66b740755 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -14,4 +14,4 @@ export * from "./UI.js";
 
 export * from "./useDayPicker.js";
 
-export { enUS as defaultLocale } from "./lib/locales.js";
+export { defaultLocale } from "./lib/locales.js";
diff --git a/src/labels/labelDayButton.ts b/src/labels/labelDayButton.ts
index f772e6b7a..b7c4be1e5 100644
--- a/src/labels/labelDayButton.ts
+++ b/src/labels/labelDayButton.ts
@@ -1,5 +1,4 @@
-import type { DateLib, LabelOptions } from "../index.js";
-import { dateLib as defaultDateLib } from "../lib/index.js";
+import { DateLib, LabelOptions } from "../lib/dateLib.js";
 import type { Modifiers } from "../types/index.js";
 
 /**
@@ -18,9 +17,9 @@ export function labelDayButton(
   modifiers: Modifiers,
   options?: LabelOptions,
   /** @ignore */
-  dateLib: DateLib = defaultDateLib
+  dateLib: DateLib = DateLib.fromOptionsDefaultLocale(options)
 ) {
-  let label = dateLib.format(date, "PPPP", options);
+  let label = dateLib.format(date, "PPPP");
   if (modifiers.today) label = `Today, ${label}`;
   if (modifiers.selected) label = `${label}, selected`;
   return label;
diff --git a/src/labels/labelGrid.ts b/src/labels/labelGrid.ts
index be9149277..ec1f65f53 100644
--- a/src/labels/labelGrid.ts
+++ b/src/labels/labelGrid.ts
@@ -1,5 +1,4 @@
-import type { LabelOptions, DateLib } from "../lib/dateLib.js";
-import { dateLib as defaultDateLib } from "../lib/index.js";
+import { DateLib, type LabelOptions } from "../lib/dateLib.js";
 
 /**
  * Return an ARIA label for the month grid, that will be announced when entering
@@ -13,9 +12,9 @@ export function labelGrid(
   date: Date,
   options?: LabelOptions,
   /** @ignore */
-  dateLib: DateLib = defaultDateLib
+  dateLib: DateLib = DateLib.fromOptionsDefaultLocale(options)
 ) {
-  return dateLib.format(date, "LLLL y", options);
+  return dateLib.format(date, "LLLL y");
 }
 
 /**
diff --git a/src/labels/labelGridcell.ts b/src/labels/labelGridcell.ts
index 87409e33a..682b4d626 100644
--- a/src/labels/labelGridcell.ts
+++ b/src/labels/labelGridcell.ts
@@ -1,6 +1,4 @@
-import type { DateLib } from "../index.js";
-import type { LabelOptions } from "../lib/dateLib.js";
-import { dateLib as defaultDateLib } from "../lib/index.js";
+import { DateLib, type LabelOptions } from "../lib/dateLib.js";
 import type { Modifiers } from "../types/index.js";
 
 /**
@@ -15,9 +13,9 @@ export function labelGridcell(
   modifiers?: Modifiers,
   options?: LabelOptions,
   /** @ignore */
-  dateLib: DateLib = defaultDateLib
+  dateLib: DateLib = DateLib.fromOptionsDefaultLocale(options)
 ) {
-  let label = dateLib.format(date, "PPPP", options);
+  let label = dateLib.format(date, "PPPP");
   if (modifiers?.today) {
     label = `Today, ${label}`;
   }
diff --git a/src/labels/labelWeekday.ts b/src/labels/labelWeekday.ts
index 807c9a400..8afea40ab 100644
--- a/src/labels/labelWeekday.ts
+++ b/src/labels/labelWeekday.ts
@@ -1,5 +1,4 @@
-import type { LabelOptions, DateLib } from "../lib/dateLib.js";
-import { dateLib as defaultDateLib } from "../lib/index.js";
+import { DateLib, type LabelOptions } from "../lib/dateLib.js";
 
 /**
  * The ARIA label for the Weekday column header.
@@ -12,7 +11,7 @@ export function labelWeekday(
   date: Date,
   options?: LabelOptions,
   /** @ignore */
-  dateLib: DateLib = defaultDateLib
+  dateLib: DateLib = DateLib.fromOptionsDefaultLocale(options)
 ): string {
-  return dateLib.format(date, "cccc", options);
+  return dateLib.format(date, "cccc");
 }
diff --git a/src/lib/dateLib.ts b/src/lib/dateLib.ts
index 9a6f044c8..5947714f9 100644
--- a/src/lib/dateLib.ts
+++ b/src/lib/dateLib.ts
@@ -1,6 +1,7 @@
 import type {
   FormatOptions as DateFnsFormatOptions,
-  Locale as DateFnsLocale
+  Locale as DateFnsLocale,
+  DateArg
 } from "date-fns";
 import { addDays } from "date-fns/addDays";
 import { addMonths } from "date-fns/addMonths";
@@ -31,6 +32,8 @@ import { startOfMonth } from "date-fns/startOfMonth";
 import { startOfWeek } from "date-fns/startOfWeek";
 import { startOfYear } from "date-fns/startOfYear";
 
+import { defaultLocale } from "./locales.js";
+
 /** The options for the {@link Formatters}. */
 export type FormatOptions = DateFnsFormatOptions;
 
@@ -42,13 +45,15 @@ export type Locale = DateFnsLocale;
 
 export type { Month as DateFnsMonth } from "date-fns";
 
+type Options = Omit<FormatOptions, "locale"> & { locale?: Locale };
+
 /**
  * The date library used by DayPicker. It's a subset of the date-fns functions
  * plus an optional Date constructor.
  *
  * Override the default date library with the `dateLib` prop.
  */
-export type DateLib = {
+export type DateLibBase = {
   /** The constructor of the date object. */
   Date?: DateConstructor | undefined;
 
@@ -143,7 +148,7 @@ export type DateLib = {
  * @private
  * @internal
  */
-export const dateLib = {
+const defaultDateLibBase: DateLibBase = {
   addDays,
   addMonths,
   addWeeks,
@@ -173,3 +178,206 @@ export const dateLib = {
   startOfWeek,
   startOfYear
 };
+
+/**
+ * The date library used by DayPicker. It's a subset of the date-fns functions
+ * with an explicit locale / formatting options.
+ *
+ * @private
+ * @internal
+ */
+export class DateLib {
+  /**
+   * Creates a new instance of the date library.
+   *
+   * @param options The formatting options for the date library.
+   * @param dateLibBaseOverrides Overrides for the date library functions.
+   */
+  constructor(
+    readonly options: Options = {},
+    dateLibBaseOverrides: Partial<DateLibBase> = {}
+  ) {
+    this.dateLibBase = {
+      ...defaultDateLibBase,
+      ...dateLibBaseOverrides
+    };
+  }
+
+  /**
+   * Creates a new instance of the date library with the default locale.
+   *
+   * @param options The formatting options for the date library.
+   */
+  static fromOptionsDefaultLocale(
+    options: FormatOptions | Options | undefined
+  ) {
+    return new DateLib({
+      ...options,
+      locale: { ...defaultLocale, ...options?.locale }
+    });
+  }
+
+  /** The options used by the date library. */
+  private readonly dateLibBase: DateLibBase;
+
+  // =====================================================================
+  // DateLib functions which are just wrappers around date-fns functions
+  // They do not require locale overrides as they are not locale-dependent
+  // =====================================================================
+
+  /** Adds the specified number of days to the given date. */
+  addDays: DateLibBase["addDays"] = (...args) =>
+    this.dateLibBase.addDays(...args);
+
+  /** Adds the specified number of months to the given date. */
+  addMonths: DateLibBase["addMonths"] = (...args) =>
+    this.dateLibBase.addMonths(...args);
+
+  /** Adds the specified number of weeks to the given date. */
+  addWeeks: DateLibBase["addWeeks"] = (...args) =>
+    this.dateLibBase.addWeeks(...args);
+
+  /** Adds the specified number of years to the given date. */
+  addYears: DateLibBase["addYears"] = (...args) =>
+    this.dateLibBase.addYears(...args);
+
+  /** Returns the number of calendar days between the given dates. */
+  differenceInCalendarDays: DateLibBase["differenceInCalendarDays"] = (
+    ...args
+  ) => this.dateLibBase.differenceInCalendarDays(...args);
+
+  /** Returns the number of calendar months between the given dates. */
+  differenceInCalendarMonths: DateLibBase["differenceInCalendarMonths"] = (
+    ...args
+  ) => this.dateLibBase.differenceInCalendarMonths(...args);
+
+  /** Returns the end of an ISO week for the given date. */
+  endOfISOWeek: DateLibBase["endOfISOWeek"] = (...args) =>
+    this.dateLibBase.endOfISOWeek(...args);
+
+  /** Returns the end of the month for the given date. */
+  endOfMonth: DateLibBase["endOfMonth"] = (...args) =>
+    this.dateLibBase.endOfMonth(...args);
+
+  /** Returns the end of the year for the given date. */
+  endOfYear: DateLibBase["endOfYear"] = (...args) =>
+    this.dateLibBase.endOfYear(...args);
+
+  /** Returns the ISO week number for the given date. */
+  getISOWeek: DateLibBase["getISOWeek"] = (...args) =>
+    this.dateLibBase.getISOWeek(...args);
+
+  /** Checks if the first date is after the second date. */
+  isAfter: DateLibBase["isAfter"] = (...args) =>
+    this.dateLibBase.isAfter(...args);
+
+  /** Checks if the first date is before the second date. */
+  isBefore: DateLibBase["isBefore"] = (...args) =>
+    this.dateLibBase.isBefore(...args);
+
+  /** Checks if the given value is a date. */
+  isDate: DateLibBase["isDate"] = (value): value is Date =>
+    this.dateLibBase.isDate(value);
+
+  /** Checks if the given dates are the same day. */
+  isSameDay: DateLibBase["isSameDay"] = (...args) =>
+    this.dateLibBase.isSameDay(...args);
+
+  /** Checks if the given dates are in the same month. */
+  isSameMonth: DateLibBase["isSameMonth"] = (...args) =>
+    this.dateLibBase.isSameMonth(...args);
+
+  /** Checks if the given dates are in the same year. */
+  isSameYear: DateLibBase["isSameYear"] = (...args) =>
+    this.dateLibBase.isSameYear(...args);
+
+  /** Returns the maximum of the given dates. */
+  max: DateLibBase["max"] = (...args) => this.dateLibBase.max(...args);
+
+  /** Returns the minimum of the given dates. */
+  min: DateLibBase["min"] = (...args) => this.dateLibBase.min(...args);
+
+  /** Sets the month for the given date. */
+  setMonth: DateLibBase["setMonth"] = (...args) =>
+    this.dateLibBase.setMonth(...args);
+
+  /** Sets the year for the given date. */
+  setYear: DateLibBase["setYear"] = (...args) =>
+    this.dateLibBase.setYear(...args);
+
+  /** Returns the start of the day for the given date. */
+  startOfDay: DateLibBase["startOfDay"] = (...args) =>
+    this.dateLibBase.startOfDay(...args);
+
+  /** Returns the start of an ISO week for the given date. */
+  startOfISOWeek: DateLibBase["startOfISOWeek"] = (...args) =>
+    this.dateLibBase.startOfISOWeek(...args);
+
+  /** Returns the start of the month for the given date. */
+  startOfMonth: DateLibBase["startOfMonth"] = (...args) =>
+    this.dateLibBase.startOfMonth(...args);
+
+  /** Returns the start of the year for the given date. */
+  startOfYear: DateLibBase["startOfYear"] = (...args) =>
+    this.dateLibBase.startOfYear(...args);
+
+  // ================================================
+  // DateLib functions which require locale overrides
+  // ================================================
+
+  /** Returns the end of the week for the given date. */
+  endOfWeek = <DateType extends Date>(date: DateArg<DateType>) => {
+    if (!this.locale) {
+      throw new Error(
+        "DateLib needs to be initialized with locale to use the `endOfWeek` function."
+      );
+    }
+
+    return endOfWeek(date, this.options);
+  };
+
+  /** Formats the given date using the specified format string. */
+  format = (date: DateArg<Date> & {}, formatStr: string) => {
+    if (!this.locale) {
+      throw new Error(
+        "DateLib needs to be initialized with locale to use the `format` function."
+      );
+    }
+
+    return format(date, formatStr, this.options);
+  };
+
+  /** Returns the week number for the given date. */
+  getWeek = (date: DateArg<Date> & {}) => {
+    if (!this.locale) {
+      throw new Error(
+        "DateLib needs to be initialized with locale to use the `getWeek` function."
+      );
+    }
+
+    return getWeek(date, this.options);
+  };
+
+  /** Returns the start of the week for the given date. */
+  startOfWeek = <DateType extends Date, ResultDate extends Date = DateType>(
+    date: DateArg<DateType>
+  ) => {
+    if (!this.locale) {
+      throw new Error(
+        "DateLib needs to be initialized with locale to use the `startOfWeek` function."
+      );
+    }
+
+    return startOfWeek(date, this.options);
+  };
+
+  /** The locale used by the date library. */
+  get locale(): Locale | undefined {
+    return this.options.locale;
+  }
+
+  /** The constructor of the date object. */
+  get Date(): DateConstructor | undefined {
+    return this.dateLibBase.Date;
+  }
+}
diff --git a/src/lib/locales.ts b/src/lib/locales.ts
index 3c0f52435..5e154069d 100644
--- a/src/lib/locales.ts
+++ b/src/lib/locales.ts
@@ -1 +1 @@
-export { enUS } from "date-fns/locale/en-US";
+export { enUS as defaultLocale } from "date-fns/locale/en-US";
diff --git a/src/selection/useMulti.test.tsx b/src/selection/useMulti.test.tsx
index 5c8fea569..f3255e7a8 100644
--- a/src/selection/useMulti.test.tsx
+++ b/src/selection/useMulti.test.tsx
@@ -1,7 +1,6 @@
-import { dateLib } from "react-day-picker/lib";
-
 import { act, renderHook } from "@/test/render";
 
+import { DateLib } from "../lib/dateLib";
 import { DayPickerProps } from "../types";
 
 import { useMulti } from "./useMulti";
@@ -16,7 +15,7 @@ describe("useMulti", () => {
       onSelect: mockOnSelect
     };
 
-    const { result } = renderHook(() => useMulti(props, dateLib));
+    const { result } = renderHook(() => useMulti(props, new DateLib()));
 
     expect(result.current.selected).toBe(selectedDates);
   });
@@ -28,7 +27,7 @@ describe("useMulti", () => {
       selected: initialSelectedDates
     };
 
-    const { result } = renderHook(() => useMulti(props, dateLib));
+    const { result } = renderHook(() => useMulti(props, new DateLib()));
 
     act(() => {
       result.current.select?.(new Date(2023, 9, 3), {}, {} as React.MouseEvent);
diff --git a/src/selection/useRange.test.tsx b/src/selection/useRange.test.tsx
index b53181333..1acf03247 100644
--- a/src/selection/useRange.test.tsx
+++ b/src/selection/useRange.test.tsx
@@ -3,7 +3,7 @@ import { DayPickerProps } from "react-day-picker/types";
 
 import { act, renderHook } from "@/test/render";
 
-import { dateLib } from "../lib";
+import { DateLib } from "../lib/dateLib";
 
 import { useRange } from "./useRange";
 
@@ -16,7 +16,7 @@ describe("useRange", () => {
     const { result } = renderHook(() =>
       useRange(
         { mode: "range", selected: initiallySelected, required: false },
-        dateLib
+        new DateLib()
       )
     );
 
@@ -31,7 +31,7 @@ describe("useRange", () => {
     const { result } = renderHook(() =>
       useRange(
         { mode: "range", selected: initiallySelected, required: false },
-        dateLib
+        new DateLib()
       )
     );
 
@@ -54,7 +54,7 @@ describe("useRange", () => {
           required: false,
           max: 5
         },
-        dateLib
+        new DateLib()
       )
     );
 
@@ -73,7 +73,7 @@ describe("useRange", () => {
     const { result } = renderHook(() =>
       useRange(
         { mode: "range", selected: undefined, required: false, min: 5 },
-        dateLib
+        new DateLib()
       )
     );
 
@@ -99,7 +99,7 @@ describe("useRange", () => {
           excludeDisabled: true,
           disabled
         },
-        dateLib
+        new DateLib()
       )
     );
 
@@ -126,7 +126,7 @@ describe("useRange", () => {
       onSelect: mockOnSelect
     };
 
-    const { result } = renderHook(() => useRange(props, dateLib));
+    const { result } = renderHook(() => useRange(props, new DateLib()));
 
     expect(result.current.selected).toBe(selectedRange);
   });
@@ -141,7 +141,7 @@ describe("useRange", () => {
       selected: initialSelectedRange
     };
 
-    const { result } = renderHook(() => useRange(props, dateLib));
+    const { result } = renderHook(() => useRange(props, new DateLib()));
 
     act(() => {
       result.current.select?.(new Date(2023, 9, 6), {}, {} as React.MouseEvent);
diff --git a/src/selection/useSingle.test.tsx b/src/selection/useSingle.test.tsx
index e1bebf050..4b280727b 100644
--- a/src/selection/useSingle.test.tsx
+++ b/src/selection/useSingle.test.tsx
@@ -1,7 +1,6 @@
-import { dateLib } from "react-day-picker/lib";
-
 import { act, renderHook } from "@/test/render";
 
+import { DateLib } from "../lib/dateLib";
 import { DayPickerProps } from "../types";
 
 import { useSingle } from "./useSingle";
@@ -16,7 +15,7 @@ describe("useSingle", () => {
       onSelect: mockOnSelect
     };
 
-    const { result } = renderHook(() => useSingle(props, dateLib));
+    const { result } = renderHook(() => useSingle(props, new DateLib()));
 
     expect(result.current.selected).toBe(selectedDate);
   });
@@ -28,7 +27,7 @@ describe("useSingle", () => {
       selected: initialSelectedDate
     };
 
-    const { result } = renderHook(() => useSingle(props, dateLib));
+    const { result } = renderHook(() => useSingle(props, new DateLib()));
 
     act(() => {
       result.current.select?.(new Date(2023, 9, 2), {}, {} as React.MouseEvent);
diff --git a/src/types/props.ts b/src/types/props.ts
index 980f62c54..853a65114 100644
--- a/src/types/props.ts
+++ b/src/types/props.ts
@@ -1,7 +1,7 @@
 import React from "react";
 
 import type { DeprecatedUI } from "../UI.js";
-import type { Locale, DateLib } from "../lib/dateLib.js";
+import type { Locale, DateLibBase } from "../lib/dateLib.js";
 
 import type {
   ClassNames,
@@ -441,7 +441,7 @@ export interface PropsBase {
    * @since 9.0.0
    * @experimental
    */
-  dateLib?: Partial<DateLib> | undefined;
+  dateLib?: Partial<DateLibBase> | undefined;
 
   /**
    * @private
diff --git a/src/useCalendar.ts b/src/useCalendar.ts
index 68992e351..8bb227196 100644
--- a/src/useCalendar.ts
+++ b/src/useCalendar.ts
@@ -75,7 +75,6 @@ export function useCalendar(
     | "today"
     | "fixedWeeks"
     | "ISOWeek"
-    | "weekStartsOn"
     | "numberOfMonths"
     | "disableNavigation"
     | "onMonthChange"
diff --git a/src/utils/addToRange.ts b/src/utils/addToRange.ts
index c46102db0..584527b39 100644
--- a/src/utils/addToRange.ts
+++ b/src/utils/addToRange.ts
@@ -1,4 +1,4 @@
-import { dateLib as defaultDateLib, type DateLib } from "../lib/index.js";
+import { DateLib } from "../lib/dateLib.js";
 import type { DateRange } from "../types/index.js";
 
 /**
@@ -18,7 +18,7 @@ export function addToRange(
   max = 0,
   required = false,
   /** @ignore */
-  dateLib: DateLib = defaultDateLib
+  dateLib: DateLib = new DateLib()
 ): DateRange | undefined {
   const { from, to } = initialRange || {};
   const { isSameDay, isAfter, isBefore } = dateLib;
diff --git a/src/utils/dateMatchModifiers.test.ts b/src/utils/dateMatchModifiers.test.ts
index d9a784c63..ea270041a 100644
--- a/src/utils/dateMatchModifiers.test.ts
+++ b/src/utils/dateMatchModifiers.test.ts
@@ -1,6 +1,6 @@
 import { addDays, subDays } from "date-fns";
 
-import { dateLib } from "../lib";
+import { DateLib } from "../lib/dateLib";
 import {
   DateAfter,
   DateBefore,
@@ -15,14 +15,14 @@ const testDay = new Date();
 
 describe("when the matcher is a boolean", () => {
   const matcher = true;
-  const result = dateMatchModifiers(testDay, [matcher], dateLib);
+  const result = dateMatchModifiers(testDay, [matcher], new DateLib());
   test("should return the boolean", () => {
     expect(result).toBe(matcher);
   });
 });
 describe("when matching the same day", () => {
   const matcher = testDay;
-  const result = dateMatchModifiers(testDay, [matcher], dateLib);
+  const result = dateMatchModifiers(testDay, [matcher], new DateLib());
   test("should return true", () => {
     expect(result).toBe(true);
   });
@@ -30,7 +30,7 @@ describe("when matching the same day", () => {
 
 describe("when matching an array of dates including the day", () => {
   const matcher = [addDays(testDay, -1), testDay, addDays(testDay, 1)];
-  const result = dateMatchModifiers(testDay, [matcher], dateLib);
+  const result = dateMatchModifiers(testDay, [matcher], new DateLib());
   test("should return true", () => {
     expect(result).toBe(true);
   });
@@ -41,7 +41,7 @@ describe("when matching date range", () => {
     from: testDay,
     to: addDays(testDay, 1)
   };
-  const result = dateMatchModifiers(testDay, [matcher], dateLib);
+  const result = dateMatchModifiers(testDay, [matcher], new DateLib());
   test("should return true", () => {
     expect(result).toBe(true);
   });
@@ -51,7 +51,7 @@ describe("when matching the day of week", () => {
   const matcher: DayOfWeek = {
     dayOfWeek: [testDay.getDay()]
   };
-  const result = dateMatchModifiers(testDay, [matcher], dateLib);
+  const result = dateMatchModifiers(testDay, [matcher], new DateLib());
   test("should return true", () => {
     expect(result).toBe(true);
   });
@@ -62,7 +62,7 @@ describe("when matching date interval (closed)", () => {
     before: addDays(testDay, 5),
     after: subDays(testDay, 3)
   };
-  const result = dateMatchModifiers(testDay, [matcher], dateLib);
+  const result = dateMatchModifiers(testDay, [matcher], new DateLib());
   test("should return true for the included day", () => {
     expect(result).toBe(true);
   });
@@ -74,22 +74,30 @@ describe("when matching date interval (open)", () => {
     after: addDays(testDay, 5)
   };
   test("should return false", () => {
-    const result = dateMatchModifiers(testDay, [matcher], dateLib);
+    const result = dateMatchModifiers(testDay, [matcher], new DateLib());
     expect(result).toBe(false);
   });
   test("should return true for the days before", () => {
-    const result = dateMatchModifiers(subDays(testDay, 8), [matcher], dateLib);
+    const result = dateMatchModifiers(
+      subDays(testDay, 8),
+      [matcher],
+      new DateLib()
+    );
     expect(result).toBe(true);
   });
   test("should return true for the days after", () => {
-    const result = dateMatchModifiers(addDays(testDay, 8), [matcher], dateLib);
+    const result = dateMatchModifiers(
+      addDays(testDay, 8),
+      [matcher],
+      new DateLib()
+    );
     expect(result).toBe(true);
   });
 });
 
 describe("when matching the date after", () => {
   const matcher: DateAfter = { after: addDays(testDay, -1) };
-  const result = dateMatchModifiers(testDay, [matcher], dateLib);
+  const result = dateMatchModifiers(testDay, [matcher], new DateLib());
   test("should return true", () => {
     expect(result).toBe(true);
   });
@@ -97,7 +105,7 @@ describe("when matching the date after", () => {
 
 describe("when matching the date before", () => {
   const matcher: DateBefore = { before: addDays(testDay, +1) };
-  const result = dateMatchModifiers(testDay, [matcher], dateLib);
+  const result = dateMatchModifiers(testDay, [matcher], new DateLib());
   test("should return true", () => {
     expect(result).toBe(true);
   });
@@ -105,7 +113,7 @@ describe("when matching the date before", () => {
 
 describe("when the matcher is a function", () => {
   const matcher = () => true;
-  const result = dateMatchModifiers(testDay, [matcher], dateLib);
+  const result = dateMatchModifiers(testDay, [matcher], new DateLib());
   test("should return the result of the function", () => {
     expect(result).toBe(true);
   });
diff --git a/src/utils/dateMatchModifiers.ts b/src/utils/dateMatchModifiers.ts
index a3caa4031..19cc198f3 100644
--- a/src/utils/dateMatchModifiers.ts
+++ b/src/utils/dateMatchModifiers.ts
@@ -1,4 +1,4 @@
-import { dateLib as defaultDateLib, type DateLib } from "../lib/dateLib.js";
+import { DateLib } from "../lib/dateLib.js";
 import type { Matcher } from "../types/index.js";
 
 import { rangeIncludesDate } from "./rangeIncludesDate.js";
@@ -33,7 +33,7 @@ import {
 export function dateMatchModifiers(
   date: Date,
   matchers: Matcher | Matcher[],
-  dateLib: DateLib = defaultDateLib
+  dateLib: DateLib = new DateLib()
 ): boolean {
   const matchersArr = !Array.isArray(matchers) ? [matchers] : matchers;
   const { isSameDay, differenceInCalendarDays, isAfter } = dateLib;
diff --git a/src/utils/rangeIncludesDate.ts b/src/utils/rangeIncludesDate.ts
index 04823f2a3..1162e3d63 100644
--- a/src/utils/rangeIncludesDate.ts
+++ b/src/utils/rangeIncludesDate.ts
@@ -1,4 +1,4 @@
-import { dateLib as defaultDateLib, type DateLib } from "../lib/index.js";
+import { DateLib } from "../lib/index.js";
 import type { DateRange } from "../types/index.js";
 
 /**
@@ -13,7 +13,7 @@ export function rangeIncludesDate(
   /** If `true`, the ends of the range are excluded. */
   excludeEnds = false,
   /** @ignore */
-  dateLib: DateLib = defaultDateLib
+  dateLib: DateLib = new DateLib()
 ): boolean {
   let { from, to } = range;
   const { differenceInCalendarDays, isSameDay } = dateLib;
@@ -41,4 +41,4 @@ export function rangeIncludesDate(
  * @deprecated Use {@link rangeIncludesDate} instead.
  */
 export const isDateInRange = (range: DateRange, date: Date) =>
-  rangeIncludesDate(range, date, false, defaultDateLib);
+  rangeIncludesDate(range, date, false, new DateLib());
diff --git a/src/utils/typeguards.test.ts b/src/utils/typeguards.test.ts
index 63ec85d2d..2572ae093 100644
--- a/src/utils/typeguards.test.ts
+++ b/src/utils/typeguards.test.ts
@@ -1,4 +1,4 @@
-import { dateLib } from "../lib/dateLib.js";
+import { DateLib } from "../lib/dateLib.js";
 import type {
   DateInterval,
   DateRange,
@@ -73,11 +73,11 @@ test("isDayOfWeekType return false for invalid DayOfWeek", () => {
 
 test("isDatesArray return true for valid array of dates", () => {
   const validDatesArray: Date[] = [new Date(), new Date()];
-  expect(isDatesArray(validDatesArray, dateLib)).toBe(true);
+  expect(isDatesArray(validDatesArray, new DateLib())).toBe(true);
 });
 
 test("isDatesArray return false for invalid array of dates", () => {
-  expect(isDatesArray([{}, {}], dateLib)).toBe(false);
-  expect(isDatesArray(null, dateLib)).toBe(false);
-  expect(isDatesArray(undefined, dateLib)).toBe(false);
+  expect(isDatesArray([{}, {}], new DateLib())).toBe(false);
+  expect(isDatesArray(null, new DateLib())).toBe(false);
+  expect(isDatesArray(undefined, new DateLib())).toBe(false);
 });