Define Enso epoch start as 15th October 1582.

distribution/lib/Standard/Base/0.0.0-dev/src/Data/Time/Date.enso

@@ -184,6 +184,8 @@ type Date
 
     ## Returns the number of week of year this date falls into.
 
+       Throws `Time_Error` for a Date that is before epoch start.
+
        Arguments:
        - locale: the locale used to define the notion of weeks of year.
          If no locale is provided, then the ISO 8601 week of year is used.
@@ -196,18 +198,23 @@ type Date
          other hand, the first day of the week is Monday, and week 1 is the week
          containing the first Thursday of the year. Therefore it is important to
          properly specify the `locale` argument.
-    week_of_year : (Locale.Locale | Nothing) -> Integer
+    week_of_year : (Locale.Locale | Nothing) -> Integer ! Time_Error
     week_of_year self locale=Nothing =
-        if locale.is_nothing then Time_Utils.get_field_as_localdate self IsoFields.WEEK_OF_WEEK_BASED_YEAR else
-            Time_Utils.week_of_year_localdate self locale.java_locale
+        self_in_epoch = Date_Time.ensure_in_epoch self
+        if locale.is_nothing then Time_Utils.get_field_as_localdate self_in_epoch IsoFields.WEEK_OF_WEEK_BASED_YEAR else
+            Time_Utils.week_of_year_localdate self_in_epoch locale.java_locale
 
     ## Returns if the date is in a leap year.
-    is_leap_year : Boolean
-    is_leap_year self = Time_Utils.is_leap_year self
+
+       Throws `Time_Error` for a Date that is before epoch start.
+    is_leap_year : Boolean ! Time_Error
+    is_leap_year self = Time_Utils.is_leap_year (Date_Time.ensure_in_epoch self)
 
     ## Returns the number of days in the year represented by this date.
-    length_of_year : Integer
-    length_of_year self = if self.is_leap_year then 366 else 365
+
+       Throws `Time_Error` for a Date that is before epoch start.
+    length_of_year : Integer ! Time_Error
+    length_of_year self = if (Date_Time.ensure_in_epoch self).is_leap_year then 366 else 365
 
     ## Returns the century of the date.
     century : Integer
@@ -219,17 +226,21 @@ type Date
     quarter self = Time_Utils.get_field_as_localdate self IsoFields.QUARTER_OF_YEAR
 
     ## Returns the number of days in the month represented by this date.
-    length_of_month : Integer
-    length_of_month self = Time_Utils.length_of_month self
+
+       Throws `Time_Error` for a Date that is before epoch start.
+    length_of_month : Integer ! Time_Error
+    length_of_month self = Time_Utils.length_of_month (Date_Time.ensure_in_epoch self)
 
     ## Returns the day of the year.
     day_of_year : Integer
     day_of_year self = Time_Utils.get_field_as_localdate self ChronoField.DAY_OF_YEAR
 
     ## Returns the day of the week.
-    day_of_week : Day_Of_Week
+
+       Throws `Time_Error` for a Date that is before epoch start.
+    day_of_week : Day_Of_Week ! Time_Error
     day_of_week self =
-        Day_Of_Week.from (Time_Utils.get_field_as_localdate self ChronoField.DAY_OF_WEEK) Day_Of_Week.Monday
+        Day_Of_Week.from (Time_Utils.get_field_as_localdate (Date_Time.ensure_in_epoch self) ChronoField.DAY_OF_WEEK) Day_Of_Week.Monday
 
     ## Returns the first date within the `Date_Period` containing self.
     start_of : Date_Period -> Date
@@ -242,6 +253,9 @@ type Date
     ## Counts workdays between self (inclusive) and the provided end date
        (exclusive).
 
+       Throws `Time_Error` if called on a Date that is before an epoch
+       start. See `Date_Time.epoch_start`.
+
        Arguments:
        - end: the end date of the interval to count workdays in.
        - holidays: dates of holidays to skip when counting workdays.
@@ -268,16 +282,17 @@ type Date
              from Standard.Base import Date
 
              example_workdays = Date.new 2020 1 1 . work_days_until (Date.new 2020 1 5)
-    work_days_until : Date -> Vector Date -> Boolean -> Integer
+    work_days_until : Date -> Vector Date -> Boolean -> Integer ! Time_Error
     work_days_until self end holidays=[] include_end_date=False =
-        if include_end_date then self.work_days_until (end + 1.day) holidays include_end_date=False else
-            weekdays = week_days_between self end
+        self_in_epoch = Date_Time.ensure_in_epoch self
+        if include_end_date then self_in_epoch.work_days_until (end + 1.day) holidays include_end_date=False else
+            weekdays = week_days_between self_in_epoch end
             ## We count holidays that occurred within the period, but not on the
                weekends (as weekend days have already been excluded from the count).
                We also need to ensure we exclude each holiday only once, even if the
                user provided it multiple times.
             overlapping_holidays = holidays.filter holiday->
-                fits_in_range self end holiday && (is_weekend holiday).not
+                fits_in_range self_in_epoch end holiday && (is_weekend holiday).not
             weekdays - overlapping_holidays.distinct.length
 
     ## ALIAS Date to Time
@@ -316,6 +331,9 @@ type Date
        For the purpose of this method, the business days are defined to be
        Monday through Friday.
 
+       Throws `Time_Error` if called on a Date that is before an epoch
+       start. See `Date_Time.epoch_start`.
+
        This method always returns a day which is a business day - if the shift
        amount is zero, the closest following business day is returned. For the
        purpose of calculating the shift, the holidays are treated as if we were
@@ -342,14 +360,15 @@ type Date
          Shift the date by 5 business days.
 
              example_shift = Date.new 2020 2 3 . add_work_days 5
-    add_work_days : Integer -> Vector Date -> Date
+    add_work_days : Integer -> Vector Date -> Date ! Time_Error
     add_work_days self days=1 holidays=[] = case days >= 0 of
+        self_in_epoch = Date_Time.ensure_in_epoch self
         True ->
             full_weeks = days.div 5
             remaining_days = days % 5
 
             # If the current day is a Saturday, the ordinal will be 6.
-            ordinal = self.day_of_week.to_integer first_day=Day_Of_Week.Monday start_at_zero=False
+            ordinal = self_in_epoch.day_of_week.to_integer first_day=Day_Of_Week.Monday start_at_zero=False
 
             ## If the current day is a Sunday, we just need to shift by one day
                to 'escape' the weekend, regardless of the overall remaining
@@ -360,14 +379,14 @@ type Date
                 if ordinal + remaining_days > 5 then 2 else 0
 
             days_to_shift = full_weeks*7 + remaining_days + additional_shift
-            end = self + days_to_shift.days
+            end = self_in_epoch + days_to_shift.days
 
             ## We have shifted the date so that weekends are taken into account,
                but other holidays may have happened during that shift period.
                Thus we may have shifted by less workdays than really desired. We
                compute the difference and if there are still remaining workdays
                to shift by, we re-run the whole shift procedure.
-            workdays = self.work_days_until end holidays include_end_date=False
+            workdays = self_in_epoch.work_days_until end holidays include_end_date=False
             diff = days - workdays
             if diff > 0 then @Tail_Call end.add_work_days diff holidays else
                 ## Otherwise we have accounted for all workdays we were asked
@@ -390,7 +409,7 @@ type Date
             remaining_days = (days + 1) % 5 - 1
 
             # If the current day is a Sunday, the ordinal will be 1.
-            ordinal = self.day_of_week.to_integer first_day=Day_Of_Week.Sunday start_at_zero=False
+            ordinal = self_in_epoch.day_of_week.to_integer first_day=Day_Of_Week.Sunday start_at_zero=False
 
             ## If we overlapped the weekend, we need to increase the shift by
                one day (our current shift already shifts us by one day, but we
@@ -403,8 +422,8 @@ type Date
                `days_to_shift` will be negative so `end` will come _before_
                `self`.
             days_to_shift = full_weeks*7 + remaining_days + additional_shift
-            end = self + days_to_shift.days
-            workdays = end.work_days_until self holidays include_end_date=False
+            end = self_in_epoch + days_to_shift.days
+            workdays = end.work_days_until self_in_epoch holidays include_end_date=False
 
             ## `days` is negative but `workdays` is positive, `diff` will be
                zero if we accounted for all days or negative if there are

distribution/lib/Standard/Base/0.0.0-dev/src/Data/Time/Date_Time.enso

@@ -12,6 +12,27 @@ polyglot java import java.lang.ArithmeticException
 
 polyglot java import org.enso.base.Time_Utils
 
+## Obtains the start of the epoch for Enso.
+
+    ? Start of the epoch
+    For Enso, start of the epoch is equal to the start of the Gregorian calendar,
+    which is on 15th October 1582. It is possible to create a Date_Time that is
+    before the start of the epoch, however, it is invalid to invoke any (Gregorian)
+    calendar related functionalities on such a Date_Time, e.g., `length_of_month` or
+    `day_of_week`. All the other operations, including `+` and `-` should succeed.
+
+epoch_start : Date_Time
+epoch_start = @Builtin_Method "Date_Time.epoch_start"
+
+## PRIVATE
+ensure_in_epoch : (Date_Time | Date) -> (Date_Time | Date) ! Time_Error
+ensure_in_epoch date =
+    datetime = case date of
+        x : Date_Time -> x
+        x : Date.Date -> x.to_date_time
+    if epoch_start <= datetime then date else Error.throw Time_Error.epoch_start
+
+
 ## ALIAS Current Time
 
    Obtains the current date-time from the system clock in the system timezone.
@@ -29,6 +50,7 @@ now = @Builtin_Method "Date_Time.now"
    second, nanosecond and timezone.
 
    Arguments:
+   - year: The year to represent, any Integer is valid.
    - month: the month-of-year to represent, from 1 (January) to 12 (December)
    - day: the day-of-month to represent, from 1 to 31 and must be valid for the
      year and month
@@ -247,27 +269,33 @@ type Date_Time
     zone : Time_Zone
     zone self = @Builtin_Method "Date_Time.zone"
 
-    ## Return the number of seconds from the Unix epoch.
+    ## Return the number of seconds from the Enso epoch. See Date_Time.epoch_start
+
+       If this Date_Time is before an epoch start, throws `Time_Error`.
 
        > Example
-         Get the current number of seconds from the Unix epoch.
+         Get the current number of seconds from the Enso epoch.
 
              from Standard.Base import Date_Time
 
              example_epoch = Date_Time.now.to_epoch_seconds
-    to_epoch_seconds : Integer
-    to_epoch_seconds self = @Builtin_Method "Date_Time.to_epoch_seconds"
+    to_epoch_seconds : Integer ! Time_Error
+    to_epoch_seconds self =
+        (Duration.between epoch_start (ensure_in_epoch self)).total_seconds.floor
 
-    ## Return the number of milliseconds from the Unix epoch.
+    ## Return the number of milliseconds from the Enso epoch. See Date_Time.epoch_start
+
+       If this Date_Time is before an epoch start, throws `Time_Error`.
 
        > Example
-         Get the current number of milliseconds from the unix epoch.
+         Get the current number of milliseconds from the Enso epoch.
 
              from Standard.Base import Date_Time
 
              example_epoch = Date_Time.now.to_epoch_milliseconds
-    to_epoch_milliseconds : Integer
-    to_epoch_milliseconds self = @Builtin_Method "Date_Time.to_epoch_milliseconds"
+    to_epoch_milliseconds : Integer ! Time_Error
+    to_epoch_milliseconds self =
+        (Duration.between epoch_start (ensure_in_epoch self)).total_milliseconds.floor
 
     ## Convert this point in time to time of day, discarding the time zone
        information.
@@ -283,6 +311,8 @@ type Date_Time
 
     ## Returns the number of week of year this date falls into.
 
+       Throws `Time_Error` for a Date_Time that is before epoch start.
+
        Arguments:
        - locale: the locale used to define the notion of weeks of year.
          If no locale is provided, then the ISO 8601 week of year is used.
@@ -295,18 +325,23 @@ type Date_Time
          other hand, the first day of the week is Monday, and week 1 is the week
          containing the first Thursday of the year. Therefore it is important to
          properly specify the `locale` argument.
-    week_of_year : (Locale.Locale | Nothing) -> Integer
+    week_of_year : (Locale.Locale | Nothing) -> Integer ! Time_Error
     week_of_year self locale=Nothing =
-        if locale.is_nothing then Time_Utils.get_field_as_zoneddatetime self IsoFields.WEEK_OF_WEEK_BASED_YEAR else
-            Time_Utils.week_of_year_zoneddatetime self locale.java_locale
+        self_in_epoch = (ensure_in_epoch self)
+        if locale.is_nothing then Time_Utils.get_field_as_zoneddatetime self_in_epoch IsoFields.WEEK_OF_WEEK_BASED_YEAR else
+            Time_Utils.week_of_year_zoneddatetime self_in_epoch locale.java_locale
 
     ## Returns if the date is in a leap year.
-    is_leap_year : Boolean
-    is_leap_year self = self.date.is_leap_year
+
+       Throws `Time_Error` for a Date_Time that is before epoch start.
+    is_leap_year : Boolean ! Time_Error
+    is_leap_year self = (ensure_in_epoch self).date.is_leap_year
 
     ## Returns the number of days in the year represented by this date.
-    length_of_year : Integer
-    length_of_year self = self.date.length_of_year
+
+       Throws `Time_Error` for a Date_Time that is before epoch start.
+    length_of_year : Integer ! Time_Error
+    length_of_year self = (ensure_in_epoch self).date.length_of_year
 
     ## Returns the century of the date.
     century : Integer
@@ -317,17 +352,19 @@ type Date_Time
     quarter self = Time_Utils.get_field_as_zoneddatetime self IsoFields.QUARTER_OF_YEAR
 
     ## Returns the number of days in the month represented by this date.
-    length_of_month : Integer
-    length_of_month self = self.date.length_of_month
+
+       Throws `Time_Error` for a Date_Time that is before epoch start.
+    length_of_month : Integer ! Time_Error
+    length_of_month self = (ensure_in_epoch self).date.length_of_month
 
     ## Returns the day of the year.
     day_of_year : Integer
     day_of_year self = Time_Utils.get_field_as_zoneddatetime self ChronoField.DAY_OF_YEAR
 
     ## Returns the day of the week.
-    day_of_week : Day_Of_Week
+    day_of_week : Day_Of_Week ! Time_Error
     day_of_week self =
-        Day_Of_Week.from (Time_Utils.get_field_as_zoneddatetime self ChronoField.DAY_OF_WEEK) Day_Of_Week.Monday
+        Day_Of_Week.from (Time_Utils.get_field_as_zoneddatetime (ensure_in_epoch self) ChronoField.DAY_OF_WEEK) Day_Of_Week.Monday
 
     ## Returns the first date within the `Time_Period` or `Date_Period`
        containing self.
@@ -398,6 +435,9 @@ type Date_Time
        For the purpose of this method, the business days are defined to be
        Monday through Friday.
 
+       Throws `Time_Error` if called on a Date_Time that is before an epoch
+       start. See `Date_Time.epoch_start`.
+
        This method always returns a day which is a business day - if the shift
        amount is zero, the closest following business day is returned. For the
        purpose of calculating the shift, the holidays are treated as if we were
@@ -426,9 +466,9 @@ type Date_Time
          Shift the date by 5 business days.
 
              example_shift = Date_Time.new 2020 2 3 11 45 . add_work_days 5
-    add_work_days : Integer -> Vector Date -> Date_Time
+    add_work_days : Integer -> Vector Date -> Date_Time ! Time_Error
     add_work_days self days=1 holidays=[] =
-        self.date.add_work_days days holidays . to_date_time self.time_of_day self.zone
+        (ensure_in_epoch self).date.add_work_days days holidays . to_date_time self.time_of_day self.zone
 
     ## Subtract the specified amount of time from this instant to get a new
        instant.

distribution/lib/Standard/Base/0.0.0-dev/src/Error/Common.enso

@@ -761,6 +761,11 @@ type Time_Error
        - error_message: The message for the error.
     Time_Error_Data error_message
 
+    ## PRIVATE
+    epoch_start : Time_Error
+    epoch_start = Time_Error_Data "Epoch start underflow"
+
+
 ## TODO Dubious constructor export
 from project.Error.Common.Unsupported_File_Type import all
 from project.Error.Common.Unsupported_File_Type export all

engine/runtime/src/main/java/org/enso/interpreter/runtime/data/EnsoDateTime.java

@@ -34,6 +34,11 @@ public EnsoDateTime(ZonedDateTime dateTime) {
     this.dateTime = dateTime;
   }
 
+  @Builtin.Method(name = "epoch_start", description = "Return the Enso start of the Epoch")
+  public static EnsoDateTime epochStart() {
+    return epochStart;
+  }
+
   @Builtin.Method(description = "Return current DateTime")
   @CompilerDirectives.TruffleBoundary
   public static EnsoDateTime now() {
@@ -162,18 +167,6 @@ public EnsoDateTime minus(Object durationObject, InteropLibrary interop)
     return new EnsoDateTime(dateTime.minus(interop.asDuration(durationObject)));
   }
 
-  @Builtin.Method(description = "Return the number of seconds from the Unix epoch.")
-  @CompilerDirectives.TruffleBoundary
-  public long toEpochSeconds() {
-    return dateTime.toEpochSecond();
-  }
-
-  @Builtin.Method(description = "Return the number of milliseconds from the Unix epoch.")
-  @CompilerDirectives.TruffleBoundary
-  public long toEpochMilliseconds() {
-    return dateTime.toInstant().toEpochMilli();
-  }
-
   @Builtin.Method(
       name = "to_localtime_builtin",
       description = "Return the localtime of this date time value.")
@@ -264,6 +257,11 @@ public final Object toDisplayString(boolean allowSideEffects) {
     return DateTimeFormatter.ISO_ZONED_DATE_TIME.format(dateTime);
   }
 
+  // 15. October 1582
+  /** 15. October 1582 in UTC timezone. Note that Java considers an epoch start 1.1.1970 UTC. */
+  private static final EnsoDateTime epochStart =
+      EnsoDateTime.create(1582, 10, 15, 0, 0, 0, 0, EnsoTimeZone.parse("UTC"));
+
   private static final DateTimeFormatter TIME_FORMAT =
       new DateTimeFormatterBuilder()
           .parseCaseInsensitive()