Remove fixed-offset formatting workaround #866 (#867)

* Remove fixed-offset formatting workaround #866

* Fix range of accepted fixed offset tz in formatter

src/impl/locale.js

@@ -183,20 +183,28 @@ class PolyDateFormatter {
 
     let z;
     if (dt.zone.universal && this.hasIntl) {
-      // Chromium doesn't support fixed-offset zones like Etc/GMT+8 in its formatter,
-      // See https://bugs.chromium.org/p/chromium/issues/detail?id=364374.
-      // 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.
-      // We can clean all this up when Chrome fixes this.
-      z = "UTC";
-      if (opts.timeZoneName) {
+      // 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. Outside of the supported range Etc/GMT-14 to Etc/GMT+12.
+      // 2. Not a whole hour, e.g. UTC+4:30.
+      const gmtOffset = -1 * (dt.offset / 60);
+      if (gmtOffset >= -14 && gmtOffset <= 12 && gmtOffset % 1 === 0) {
+        z = gmtOffset >= 0 ? `Etc/GMT+${gmtOffset}` : `Etc/GMT${gmtOffset}`;
         this.dt = dt;
       } else {
-        this.dt = dt.offset === 0 ? dt : DateTime.fromMillis(dt.ts + dt.offset * 60 * 1000);
+        // 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.
+        z = "UTC";
+        if (opts.timeZoneName) {
+          this.dt = dt;
+        } else {
+          this.dt = dt.offset === 0 ? dt : DateTime.fromMillis(dt.ts + dt.offset * 60 * 1000);
+        }
       }
     } else if (dt.zone.type === "local") {
       this.dt = dt;

test/datetime/format.test.js

@@ -314,8 +314,14 @@ test("DateTime#toLocaleString() shows things in the right fixed-offset zone", ()
   expect(dt.setZone("UTC-8").toLocaleString(DateTime.DATETIME_SHORT)).toBe("5/25/1982, 1:23 AM");
 });
 
-test("DateTime#toLocaleString() does the best it can with a fixed-offset zone when showing the zone", () => {
+test("DateTime#toLocaleString() shows things in the right fixed-offset zone when showing the zone", () => {
   expect(dt.setZone("UTC-8").toLocaleString(DateTime.DATETIME_FULL)).toBe(
+    "May 25, 1982, 1:23 AM GMT-8"
+  );
+});
+
+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, 9:23 AM UTC"
   );
 });