Bring custom time zones in line with custom calendars

This makes a small change to the constructor of Temporal.TimeZone so that
it doesn't accept any arbitrary ID, only a recognized built-in one, to
bring it in line with calendars.

See: #847
See: #300

docs/timezone.md

@@ -14,8 +14,13 @@ Since `Temporal.Instant` and `Temporal.PlainDateTime` do not contain any time zo
 For specialized applications where you need to do calculations in a time zone that is not built in, you can implement a custom time zone.
 There are two ways to do this.
 
-The recommended way is to create a class inheriting from `Temporal.TimeZone`, call `super()` in the constructor with a time zone identifier string, and implement the methods `getOffsetNanosecondsFor()` and `getPossibleInstantsFor()`, and optionally `getNextTransition()` and `getPreviousTransition()`.
-You don't need to implement any other methods because other properties and methods will use the base class's default implementations.
+The recommended way is to create a class inheriting from `Temporal.TimeZone`.
+You must use one of the built-in time zones as the "base time zone".
+In the class's constructor, call `super()` with the identifier of the base time zone.
+The class must override `toString()` to return its own identifier.
+Overriding `getOffsetNanosecondsFor()`, `getPossibleInstantsFor()`, `getNextTransition()`, and `getPreviousTransition()` is optional.
+If you don't override the optional members, then they will behave as in the base time zone.
+You don't need to override any other methods such as `getOffsetStringFor()` because they will call `getOffsetNanosecondsFor()`, `getPossibleInstantsFor()`, and `toString()` internally.
 
 The other, more difficult, way to create a custom time zone is to create a plain object implementing the `Temporal.TimeZone` protocol, without subclassing.
 The object must have at least `getOffsetNanosecondsFor()`, `getPossibleInstantsFor()`, and `toString()` methods.

polyfill/lib/timezone.mjs

@@ -19,18 +19,9 @@ import {
   SetSlot
 } from './slots.mjs';
 
-import * as REGEX from './regex.mjs';
-const OFFSET = new RegExp(`^${REGEX.offset.source}$`);
-const IANA_NAME = new RegExp(`^${REGEX.timeZoneID.source}$`);
-
 export class TimeZone {
   constructor(timeZoneIdentifier) {
-    if (new.target === TimeZone) {
-      timeZoneIdentifier = ES.GetCanonicalTimeZoneIdentifier(timeZoneIdentifier);
-    }
-    if (!OFFSET.exec(timeZoneIdentifier) && !IANA_NAME.exec(timeZoneIdentifier)) {
-      throw new RangeError(`invalid time zone identifier ${timeZoneIdentifier}`);
-    }
+    timeZoneIdentifier = ES.GetCanonicalTimeZoneIdentifier(timeZoneIdentifier);
     CreateSlots(this);
     SetSlot(this, TIMEZONE_ID, timeZoneIdentifier);
 

polyfill/test/regex.mjs

@@ -589,12 +589,16 @@ describe('fromString regex', () => {
   });
 
   describe('time zone ID', () => {
-    function makeCustomTimeZone(id) {
-      return class extends Temporal.TimeZone {
-        constructor() {
-          super(id);
-        }
+    let oldTemporalTimeZoneFrom = Temporal.TimeZone.from;
+    let fromCalledWith;
+    before(() => {
+      Temporal.TimeZone.from = function (item) {
+        fromCalledWith = item;
+        return new Temporal.TimeZone('UTC');
       };
+    });
+    function testTimeZoneID(id) {
+      return Temporal.ZonedDateTime.from(`1970-01-01T00:00[${id}]`);
     }
     describe('valid', () => {
       [
@@ -616,9 +620,8 @@ describe('fromString regex', () => {
         'Etc/GMT+12'
       ].forEach((id) => {
         it(id, () => {
-          const Custom = makeCustomTimeZone(id);
-          const tz = new Custom();
-          equal(tz.id, id);
+          testTimeZoneID(id);
+          equal(fromCalledWith, id);
         });
       });
     });
@@ -641,11 +644,13 @@ describe('fromString regex', () => {
         'Foo/Etc/GMT-8'
       ].forEach((id) => {
         it(id, () => {
-          const Custom = makeCustomTimeZone(id);
-          throws(() => new Custom(), RangeError);
+          throws(() => testTimeZoneID(id), RangeError);
         });
       });
     });
+    after(() => {
+      Temporal.TimeZone.from = oldTemporalTimeZoneFrom;
+    });
   });
 
   describe('calendar ID', () => {

polyfill/test/usertimezone.mjs

@@ -18,7 +18,10 @@ describe('Userland time zone', () => {
   describe('Trivial subclass', () => {
     class CustomUTCSubclass extends Temporal.TimeZone {
       constructor() {
-        super('Etc/Custom/UTC_Subclass');
+        super('UTC');
+      }
+      toString() {
+        return 'Etc/Custom/UTC_Subclass';
       }
       getOffsetNanosecondsFor(/* instant */) {
         return 0;
@@ -205,7 +208,10 @@ describe('Userland time zone', () => {
   describe('sub-minute offset', () => {
     class SubminuteTimeZone extends Temporal.TimeZone {
       constructor() {
-        super('Custom/Subminute');
+        super('-00:00:01.111111111');
+      }
+      toString() {
+        return 'Custom/Subminute';
       }
       getOffsetNanosecondsFor() {
         return -1111111111;

spec/timezone.html

@@ -25,21 +25,14 @@ <h1>Temporal.TimeZone ( _identifier_ )</h1>
       <emu-alg>
         1. If NewTarget is *undefined*, then
           1. Throw a *TypeError* exception.
-        1. Let _identifier_ be ? ToString(_identifier_).
-        1. If _identifier_ does not satisfy the syntax of a |TemporalTimeZoneIdentifier| (see <emu-xref href="#sec-temporal-iso8601grammar"></emu-xref>), then
-          1. Throw a *RangeError* exception.
-        1. Let _sign_, _hour_, _minute_, and _id_ be the parts of _identifier_ produced respectively by the |TimeZoneUTCOffsetSign|, |TimeZoneUTCOffsetHour|, |TimeZoneUTCOffsetMinute| and |TimeZoneIANAName| productions, or *undefined* if not present.
-        1. If _hour_ is not *undefined*, then
-          1. Assert: _sign_ is not *undefined*.
-          1. Set _hour_ to ! ToInteger(_hour_).
-          1. If _sign_ = *"-"*, then
-            1. Set _hour_ to −1 × _hour_.
-          1. If _minute_ is not *undefined*, then
-            1. Set _minute_ to ! ToInteger(_minute_).
-          1. <mark>Do something with the offset.</mark>
-        1. If ! IsValidTimeZoneName(_id_) is *false*, then
-          1. Throw a *TypeError* exception.
-        1. Let _canonical_ be ! CanonicalizeTimeZoneName(_identifier_).
+        1. Set _identifier_ to ? ToString(_identifier_).
+        1. If _identifier_ satisfies the syntax of a |TimeZoneNumericUTCOffset| (see <emu-xref href="#sec-temporal-iso8601grammar"></emu-xref>), then
+          1. Let _offsetNanoseconds_ be ? ParseTimeZoneOffsetString(_identifier_).
+          1. Let _canonical_ be ? FormatTimeZoneOffsetString(_offsetNanoseconds_).
+        1. Else,
+          1. If ! IsValidTimeZoneName(_id_) is *false*, then
+            1. Throw a *RangeError* exception.
+          1. Let _canonical_ be ! CanonicalizeTimeZoneName(_identifier_).
         1. Return ? CreateTemporalTimeZone(_canonical_, NewTarget).
       </emu-alg>
     </emu-clause>