Substitute timezone name for fixed offset timezones

src/impl/locale.js

@@ -197,9 +197,13 @@ class PolyNumberFormatter {
 class PolyDateFormatter {
   constructor(dt, intl, opts) {
     this.opts = opts;
+    this.originalZone = undefined;
 
     let z = undefined;
-    if (dt.zone.isUniversal) {
+    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) {
       // 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.
@@ -212,25 +216,23 @@ class PolyDateFormatter {
         z = offsetZ;
         this.dt = dt;
       } else {
-        // Not all fixed-offset zones like Etc/+4:30 are present in tzdata.
-        // So we have to make do. Two cases:
-        // 1. The format options tell us to show the zone. We can't do that, so the best
-        // we can do is format the date in UTC.
-        // 2. The format options don't tell us to show the zone. Then we can adjust them
-        // the time and tell the formatter to show it to us in UTC, so that the time is right
-        // and the bad zone doesn't show up.
+        // Not all fixed-offset zones like Etc/+4:30 are present in tzdata so
+        // we manually apply the offset and substitute the zone as needed.
         z = "UTC";
-        if (opts.timeZoneName) {
-          this.dt = dt;
-        } else {
-          this.dt = dt.offset === 0 ? dt : DateTime.fromMillis(dt.ts + dt.offset * 60 * 1000);
-        }
+        this.dt = dt.offset === 0 ? dt : dt.setZone("UTC").plus({ minutes: dt.offset });
+        this.originalZone = dt.zone;
       }
     } else if (dt.zone.type === "system") {
       this.dt = dt;
-    } else {
+    } else if (dt.zone.type === "iana") {
       this.dt = dt;
       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.
+      z = "UTC";
+      this.dt = dt.offset === 0 ? dt : dt.setZone("UTC").plus({ minutes: dt.offset });
+      this.originalZone = dt.zone;
     }
 
     const intlOpts = { ...this.opts };
@@ -239,11 +241,35 @@ class PolyDateFormatter {
   }
 
   format() {
+    if (this.originalZone) {
+      // If we have to substitute in the actual zone name, we have to use
+      // formatToParts so that the timezone can be replaced.
+      return this.formatToParts()
+        .map(({ value }) => value)
+        .join("");
+    }
     return this.dtf.format(this.dt.toJSDate());
   }
 
   formatToParts() {
-    return this.dtf.formatToParts(this.dt.toJSDate());
+    const parts = this.dtf.formatToParts(this.dt.toJSDate());
+    if (this.originalZone) {
+      return parts.map((part) => {
+        if (part.type === "timeZoneName") {
+          const offsetName = this.originalZone.offsetName(this.dt.ts, {
+            locale: this.dt.locale,
+            format: this.opts.timeZoneName,
+          });
+          return {
+            ...part,
+            value: offsetName,
+          };
+        } else {
+          return part;
+        }
+      });
+    }
+    return parts;
   }
 
   resolvedOptions() {

test/datetime/format.test.js

@@ -398,7 +398,13 @@ test("DateTime#toLocaleString() shows things with UTC if fixed-offset zone with
 
 test("DateTime#toLocaleString() does the best it can with unsupported fixed-offset zone when showing the zone", () => {
   expect(dt.setZone("UTC+4:30").toLocaleString(DateTime.DATETIME_FULL)).toBe(
-    "May 25, 1982 at 9:23 AM UTC"
+    "May 25, 1982 at 1:53 PM UTC+4:30"
+  );
+});
+
+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"
   );
 });