Normative: Allow implementations to report their time zone as a UTC o…

…ffset

This aligns with ECMA-262, and therefore removes the DefaultTimeZone override.

spec/annexes.html

@@ -15,9 +15,6 @@ <h1>Implementation Dependent Behaviour</h1>
         <li>
           The default locale (<emu-xref href="#sec-defaultlocale"></emu-xref>)
         </li>
-        <li>
-          The default time zone (<emu-xref href="#sup-defaulttimezone"></emu-xref>)
-        </li>
         <li>
           The set of available locales for each constructor (<emu-xref href="#sec-internal-slots"></emu-xref>)
         </li>

spec/datetimeformat.html

@@ -679,7 +679,7 @@ <h1>Properties of Intl.DateTimeFormat Instances</h1>
       <li>[[Locale]] is a String value with the language tag of the locale whose localization is used for formatting.</li>
       <li>[[Calendar]] is a String value representing the <a href="https://unicode.org/reports/tr35/#UnicodeCalendarIdentifier">Unicode Calendar Identifier</a> used for formatting.</li>
       <li>[[NumberingSystem]] is a String value representing the <a href="https://unicode.org/reports/tr35/#UnicodeNumberSystemIdentifier">Unicode Number System Identifier</a> used for formatting.</li>
-      <li>[[TimeZone]] is a String value that is a time zone identifier from the IANA Time Zone Database used for formatting.</li>
+      <li>[[TimeZone]] is a String value used for formatting that is either a time zone identifier from the IANA Time Zone Database or a UTC offset in ISO 8601 extended format.</li>
       <li>[[Weekday]], [[Era]], [[Year]], [[Month]], [[Day]], [[DayPeriod]], [[Hour]], [[Minute]], [[Second]], [[TimeZoneName]] are each either *undefined*, indicating that the component is not used for formatting, or one of the String values given in <emu-xref href="#table-datetimeformat-components"></emu-xref>, indicating how the component should be presented in the formatted output.</li>
       <li>[[FractionalSecondDigits]] is either *undefined* or a positive, non-zero integer Number value indicating the fraction digits to be used for fractional seconds. Numbers will be rounded or padded with trailing zeroes if necessary.</li>
       <li>[[HourCycle]] is a String value indicating whether the 12-hour format (*"h11"*, *"h12"*) or the 24-hour format (*"h23"*, *"h24"*) should be used. *"h11"* and *"h23"* start with hour 0 and go up to 11 and 23 respectively. *"h12"* and *"h24"* start with hour 1 and go up to 12 and 24. [[HourCycle]] is only used when [[Hour]] is not *undefined*.</li>
@@ -1150,7 +1150,11 @@ <h1>
       </dl>
 
       <emu-alg>
-        1. Let _offsetNs_ be GetNamedTimeZoneOffsetNanoseconds(_timeZoneIdentifier_, _epochNs_).
+        1. If IsTimeZoneOffsetString(_timeZoneIdentifier_) is *true*, then
+          1. Let _offsetNs_ be ParseTimeZoneOffsetString(_timeZoneIdentifier_).
+        1. Else,
+          1. Assert: IsValidTimeZoneName(_timeZoneIdentifier_) is *true*.
+          1. Let _offsetNs_ be GetNamedTimeZoneOffsetNanoseconds(_timeZoneIdentifier_, _epochNs_).
         1. Let _tz_ be ℝ(_epochNs_) + _offsetNs_.
         1. If _calendar_ is *"gregory"*, then
           1. Return a record with fields calculated from _tz_ according to <emu-xref href="#table-datetimeformat-tolocaltime-record"></emu-xref>.

spec/locales-currencies-tz.html

@@ -182,7 +182,7 @@ <h1>IsValidTimeZoneName ( _timeZone_ )</h1>
     </emu-clause>
 
     <emu-note>
-      Any value returned from DefaultTimeZone must be recognized as valid.
+      Any value returned from DefaultTimeZone that is not recognized as valid by IsTimeZoneOffsetString must be recognized as valid by IsValidTimeZoneName.
     </emu-note>
 
     <emu-clause id="sec-canonicalizetimezonename" type="abstract operation">
@@ -203,19 +203,6 @@ <h1>
       </emu-alg>
     </emu-clause>
 
-    <emu-clause id="sup-defaulttimezone" oldids="sec-defaulttimezone" type="implementation-defined abstract operation">
-      <h1>DefaultTimeZone ( ): a String</h1>
-
-      <dl class="header">
-        <dt>description</dt>
-        <dd>It returns a String value representing the host environment's current time zone, which is a valid (<emu-xref href="#sec-isvalidtimezonename"></emu-xref>) and canonicalized (<emu-xref href="#sec-canonicalizetimezonename"></emu-xref>) time zone name.</dd>
-      </dl>
-
-      <p>
-        This definition supersedes the definition provided in es2024, <emu-xref href="#sec-defaulttimezone"></emu-xref>.
-      </p>
-    </emu-clause>
-
     <emu-clause id="sec-availablecanonicaltimezones" type="implementation-defined abstract operation">
       <h1>
         AvailableCanonicalTimeZones (