Support custom zone formatting

src/impl/locale.js

@@ -1,4 +1,4 @@
-import { padStart, roundTo, hasRelative } from "./util.js";
+import { padStart, roundTo, hasRelative, formatOffset } from "./util.js";
 import * as English from "./english.js";
 import Settings from "../settings.js";
 import DateTime from "../datetime.js";
@@ -203,7 +203,7 @@ class PolyDateFormatter {
     if (this.opts.timeZone) {
       // Don't apply any workarounds if a timeZone is explicitly provided in opts
       this.dt = dt;
-    } else if (dt.zone.isUniversal) {
+    } else if (dt.zone.type === "fixed") {
       // UTC-8 or Etc/UTC-8 are not part of tzdata, only Etc/GMT+8 and the like.
       // That is why fixed-offset TZ is set to that unless it is:
       // 1. Representing offset 0 when UTC is used to maintain previous behavior and does not become GMT.
@@ -229,9 +229,9 @@ class PolyDateFormatter {
       z = dt.zone.name;
     } else {
       // Custom zones can have any offset / offsetName so we just manually
-      // apply the offset and substitute the offsetName as needed.
+      // apply the offset and substitute the zone as needed.
       z = "UTC";
-      this.dt = dt.offset === 0 ? dt : dt.setZone("UTC").plus({ minutes: dt.offset });
+      this.dt = dt.setZone("UTC").plus({ minutes: dt.offset });
       this.originalZone = dt.zone;
     }
 

test/datetime/format.test.js

@@ -1,6 +1,6 @@
 /* global test expect */
 
-import { DateTime } from "../../src/luxon";
+import { DateTime, Zone } from "../../src/luxon";
 
 const dtMaker = () =>
     DateTime.fromObject(
@@ -20,6 +20,50 @@ const dtMaker = () =>
   dt = dtMaker(),
   invalid = DateTime.invalid("because");
 
+class CustomZone extends Zone {
+  constructor(name, offset) {
+    super();
+    this._name = name;
+    this._offset = offset;
+  }
+
+  get isUniversal() {
+    return true;
+  }
+
+  get isValid() {
+    return true;
+  }
+
+  get name() {
+    return this._name;
+  }
+
+  get type() {
+    return "custom";
+  }
+
+  equals(zone) {
+    return zone instanceof CustomZone && zone._name === this._name && zone._offset === this._offset;
+  }
+
+  offset(_ms) {
+    return this._offset;
+  }
+
+  offsetName(_ms, { format }) {
+    if (format === "short") {
+      return this._name.substring(0, 4);
+    } else {
+      return this._name;
+    }
+  }
+
+  formatOffset(...args) {
+    return FixedOffsetZone.prototype.formatOffset(...args);
+  }
+}
+
 //------
 // #toJSON()
 //------
@@ -403,8 +447,24 @@ test("DateTime#toLocaleString() does the best it can with unsupported fixed-offs
 });
 
 test("DateTime#toLocaleString() does the best it can with unsupported fixed-offset zone with timeStyle full", () => {
-  expect(dt.setZone("UTC+4:30").toLocaleString({ timeStyle: "full" })).toBe(
-    "1:53:54 PM UTC+4:30"
+  expect(dt.setZone("UTC+4:30").toLocaleString({ timeStyle: "full" })).toBe("1:53:54 PM UTC+4:30");
+});
+
+test("DateTime#toLocaleString() shows things in the right custom zone", () => {
+  expect(dt.setZone(new CustomZone("CUSTOM", 30)).toLocaleString(DateTime.DATETIME_SHORT)).toBe(
+    "5/25/1982, 9:53 AM"
+  );
+});
+
+test("DateTime#toLocaleString() shows things in the right custom zone when showing the zone", () => {
+  expect(dt.setZone(new CustomZone("CUSTOM", 30)).toLocaleString(DateTime.DATETIME_FULL)).toBe(
+    "May 25, 1982 at 9:53 AM CUST"
+  );
+});
+
+test("DateTime#toLocaleString() shows things in the right custom zone with timeStyle full", () => {
+  expect(dt.setZone(new CustomZone("CUSTOM", 30)).toLocaleString({ timeStyle: "full" })).toBe(
+    "9:53:54 AM CUSTOM"
   );
 });