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Use Etc/Unknown (like ICU does) to handle the case when the OS's time zone is unrecognized by ECMAScript #25

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Use Etc/Unknown (like ICU does) to handle the case when the OS's time zone is unrecognized by ECMAScript #25

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40 changes: 40 additions & 0 deletions spec/intl.html
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Expand Up @@ -52,6 +52,46 @@ <h1>Time Zone Identifiers</h1>
</p>
</emu-clause>

<emu-clause id="sup-availablenamedtimezoneidentifiers" oldids="sec-availabletimezones" type="implementation-defined abstract operation">
<h1>AvailableNamedTimeZoneIdentifiers ( ): a List of Time Zone Identifier Records</h1>
<dl class="header">
<dt>description</dt>
<dd>
Its result describes all available named time zone identifiers in this implementation, as well as the primary time zone identifier corresponding to each available named time zone identifier.
The List is sorted by [[Identifier]] of each Time Zone Identifier Record.
</dd>
<dt>redefinition</dt>
<dd>true</dd>
</dl>
<emu-alg>
1. Let _identifiers_ be a List containing the String value of each Zone or Link name in the IANA Time Zone Database that is supported by the implementation.
1. Assert: No element of _identifiers_ is an ASCII-case-insensitive match for any other element.
1. Assert: Every element of _identifiers_ identifies a Zone or Link name in the IANA Time Zone Database.
1. <ins>Assert: No element of _identifiers_ is an ASCII-case-insensitive match for *"Etc/Unknown"*.</ins>
1. <ins>Append *"Etc/Unknown"* to _identifiers_.</ins>
1. Set _identifiers_ to SortStringListByCodeUnit(_identifiers_).
1. Let _result_ be a new empty List.
1. For each element _identifier_ of _identifiers_, do
1. Let _primary_ be _identifier_.
1. If _identifier_ is a non-primary time zone identifier in this implementation and _identifier_ is <del>not *"UTC"*</del><ins>neither *"UTC"* nor *"Etc/Unknown"*</ins>, then
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1. Set _primary_ to the name of the primary time zone identifier that _identifier_ resolves to, according to the rules for resolving Link names in the IANA Time Zone Database.
1. NOTE: If _identifier_ resolves to another non-primary time zone identifier, then the implementation must continue resolution of the entire chain until its terminal primary time zone identifier.
1. If _primary_ is one of *"Etc/UTC"*, *"Etc/GMT"*, or *"GMT"*, set _primary_ to *"UTC"*.
1. Let _record_ be the Time Zone Identifier Record { [[Identifier]]: _identifier_, [[PrimaryIdentifier]]: _primary_ }.
1. Append _record_ to the end of _result_.
1. Assert: One element of _result_ is the Time Zone Identifier Record { [[Identifier]]: *"UTC"*, [[PrimaryIdentifier]]: *"UTC"* }.
1. Return _result_.
</emu-alg>

<emu-note>
Time zone identifiers in the IANA Time Zone Database can change over time.
At a minimum, it is recommended that implementations limit changes to the result of AvailableNamedTimeZoneIdentifiers to the changes allowed by GetAvailableNamedTimeZoneIdentifier, for the lifetime of the surrounding agent.
Due to the complexity of supporting these recommendations, it is recommended that the result of AvailableNamedTimeZoneIdentifiers remains the same for the lifetime of the surrounding agent.
</emu-note>

<p>This definition supersedes the definition provided in <emu-xref href="#sec-availablenamedtimezoneidentifiers"></emu-xref>.</p>
</emu-clause>

<emu-clause id="sup-temporal.zoneddatetime.prototype.tolocalestring">
<h1>Temporal.ZonedDateTime.prototype.toLocaleString ( [ _locales_ [ , _options_ ] ] )</h1>
<p>This definition supersedes the definition provided in <emu-xref href="#sec-temporal.zoneddatetime.prototype.tolocalestring"></emu-xref>.</p>
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70 changes: 69 additions & 1 deletion spec/timezone.html
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Expand Up @@ -7,7 +7,7 @@ <h1>Amendments to the ECMAScript® 2024 Language Specification</h1>
<emu-note type="editor">
<p>
This section lists amendments which must be made to <a href="https://tc39.es/ecma262/">ECMA-262, the ECMAScript® 2024 Language Specification</a>.
The changes below are limited to the SystemTimeZoneIdentifier abstract operation, the %Temporal.TimeZone% built-in object, and the abstract operations related to that object.
The changes below are limited to the SystemTimeZoneIdentifier and AvailableNamedTimeZoneIdentifiers abstract operations, the %Temporal.TimeZone% built-in object, and the abstract operations related to that object.
Text to be added is marked <ins>like this</ins>, and text to be deleted is marked <del>like this</del>.
</p>
<p>
Expand All @@ -27,6 +27,7 @@ <h1>SystemTimeZoneIdentifier ( ): a String</h1>
<emu-alg>
1. If the implementation only supports the UTC time zone, return *"UTC"*.
1. Let _systemTimeZoneString_ be the String representing the host environment's current time zone, either a named time zone identifier or a UTC offset string.
1. <ins>Assert: _systemTimeZoneString_ is *"Etc/Unknown"* if the host environment's current time zone cannot be determined or is unrecognized by the ECMAScript implementation.</ins>
1. If IsTimeZoneOffsetString(_systemTimeZoneString_) is *true*, return <del>_systemTimeZoneString_</del><ins>CanonicalizeTimeZoneOffsetString(_systemTimeZoneString_)</ins>.
1. Assert: _systemTimeZoneString_ is the value of the [[PrimaryIdentifier]] field of at least one Time Zone Identifier Record returned by AvailableNamedTimeZoneIdentifiers().
1. Return _systemTimeZoneString_.
Expand All @@ -39,6 +40,73 @@ <h1>SystemTimeZoneIdentifier ( ): a String</h1>
</p>
<p>For example, if the host environment is a browser on a system where the user has chosen US Eastern Time as their time zone, SystemTimeZoneIdentifier returns *"America/New_York"*.</p>
</emu-note>

<ins class="block">
<emu-clause id="sec-unknown-time-zone">
<h1>Unknown Time Zone</h1>

<p>
ECMAScript implementations must make a best effort in SystemTimeZoneIdentifier to return the available named time zone identifier that is most closely associated with the host environment's time zone.
However, in some cases the host environment's time zone cannot be determined or is not recognized, for example if the host environment's time zone information is updated with a new identifier that the ECMAScript implementation does not yet include.
In those cases, the <dfn>Unknown time zone</dfn> is used.
This time zone behaves identically to the UTC time zone, except:
</p>
<ul>
<li>
Its identifier is *"Etc/Unknown"*, as defined in <a href="https://www.unicode.org/reports/tr35/tr35.html#Time_Zone_Identifiers">Unicode Technical Standard #35 Part 1 Core, Time Zone Identifiers</a>.
*"Etc/Unknown"* must be a primary time zone identifier.
For time zone aware implementations, this is the only available named time zone identifier that is not required to exist in the IANA Time Zone Database.
</li>
<li>
An ECMAScript implementation that includes the ECMA-402 Internationalization API is recommended to provide a localized name for the Unknown time zone that is different from the localized name of all other available named time zones.
</li>
</ul>
</p>
These differences help programmers and/or end users recognize that a problem exists which may require action to fix, without causing ECMAScript programs to fail whenever SystemTimeZoneIdentifier is called.
</p>
</p>
All implementations that support local political rules for any time zones, including implementations that will never return *"Etc/Unknown*" from SystemTimeZoneIdentifier because they share the host environment's list of available named time zone identifiers, must still accept *"Etc/Unknown"* as an available named time zone identifier, for example in the %Temporal.TimeZone% constructor.
</p>
<emu-note>
<p>
</p>
</emu-note>
</emu-clause>
</ins>
</emu-clause>

<emu-clause id="sec-availablenamedtimezoneidentifiers" type="implementation-defined abstract operation">
<h1>AvailableNamedTimeZoneIdentifiers ( ): a List of Time Zone Identifier Records</h1>
<dl class="header">
<dt>description</dt>
<dd>
Its result describes all available named time zone identifiers in this implementation, as well as the primary time zone identifier corresponding to each available identifier.
The List is sorted by [[Identifier]] of each Time Zone Identifier Record.
</dd>
</dl>
<p>
Time zone aware implementations, including all implementations that implement the ECMA-402 Internationalization API, must implement the AvailableNamedTimeZoneIdentifiers abstract operation as specified in the ECMA-402 specification.
For implementations that are not time zone aware, the following specification is used.
</p>
<emu-alg>
1. If the implementation does not include local political rules for any time zones, then
1. Return « the Time Zone Identifier Record { [[Identifier]]: *"UTC"*, [[PrimaryIdentifier]]: *"UTC"* } ».
1. Let _identifiers_ be the List of String values representing time zones supported by the implementation.
1. Assert: No element of _identifiers_ is an ASCII-case-insensitive match for any other element.
1. <ins>Assert: No element of _identifiers_ is an ASCII-case-insensitive match for *"Etc/Unknown"*.</ins>
1. <ins>Append *"Etc/Unknown"* to _identifiers_.</ins>
1. Set _identifiers_ to SortStringListByCodeUnit(_identifiers_).
1. Let _result_ be a new empty List.
1. For each element _identifier_ of _identifiers_, do
1. Let _primary_ be _identifier_.
1. If _identifier_ is a non-primary time zone identifier in this implementation and _identifier_ is <del>not *"UTC"*</del><ins>neither *"UTC"* nor *"Etc/Unknown"*</ins>, then
1. Set _primary_ to the primary time zone identifier that _identifier_ resolves to in this implementation.
1. NOTE: An implementation may need to resolve _identifier_ iteratively to obtain the primary time zone identifier.
1. Let _record_ be the Time Zone Identifier Record { [[Identifier]]: _identifier_, [[PrimaryIdentifier]]: _primary_ }.
1. Append _record_ to the end of _result_.
1. Assert: One element of _result_ is the Time Zone Identifier Record { [[Identifier]]: *"UTC"*, [[PrimaryIdentifier]]: *"UTC"* }.
1. Return _result_.
</emu-alg>
</emu-clause>

<emu-clause id="sec-temporal-timezone-objects">
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