diff --git a/polyfill/lib/regex.mjs b/polyfill/lib/regex.mjs
index e39504b098..704582dbfa 100644
--- a/polyfill/lib/regex.mjs
+++ b/polyfill/lib/regex.mjs
@@ -8,7 +8,7 @@ const yearpart = /(?:[+-\u2212]\d{6}|\d{4})/;
 const datesplit = new RegExp(`(${yearpart.source})(?:-(\\d{2})-(\\d{2})|(\\d{2})(\\d{2}))`);
 const timesplit = /(\d{2})(?::(\d{2})(?::(\d{2})(?:[.,](\d{1,9}))?)?|(\d{2})(?:(\d{2})(?:[.,](\d{1,9}))?)?)?/;
 export const offset = /([+-\u2212])([0-2][0-9])(?::?([0-5][0-9]))?/;
-const zonesplit = new RegExp(`(?:([zZ])|(?:${offset.source}?(?:\\[(${timeZoneID.source})\\])?))`);
+const zonesplit = new RegExp(`(?:([zZ])|(?:${offset.source})?(?:\\[(${timeZoneID.source})\\])?)`);
 const calendar = new RegExp(`\\[c=(${calendarID.source})\\]`);
 
 export const instant = new RegExp(
diff --git a/polyfill/test/regex.mjs b/polyfill/test/regex.mjs
index d67c822a81..af93d476b6 100644
--- a/polyfill/test/regex.mjs
+++ b/polyfill/test/regex.mjs
@@ -37,7 +37,7 @@ describe('fromString regex', () => {
       generateTest(`1976-11-18${timeSep}15:23`, 'Z', [1976, 11, 18, 15, 23, 30, 123, 456, 789])
     );
     // Time zone with bracketed name
-    ['+01:00', '+01', '+0100'].forEach((zoneString) =>
+    ['+01:00', '+01', '+0100', ''].forEach((zoneString) =>
       generateTest('1976-11-18T15:23', `${zoneString}[Europe/Vienna]`, [1976, 11, 18, 14, 23, 30, 123, 456, 789])
     );
     // Time zone with only offset
@@ -101,6 +101,7 @@ describe('fromString regex', () => {
     [
       '+0100[Europe/Vienna]',
       '+01:00[Europe/Vienna]',
+      '[Europe/Vienna]',
       '+01:00[Custom/Vienna]',
       '-0400',
       '-04:00',
@@ -132,7 +133,7 @@ describe('fromString regex', () => {
     test('1976-11-18T15', [1976, 11, 18, 15]);
     test('1976-11-18', [1976, 11, 18]);
     // Representations with calendar
-    ['', 'Z', '+01:00[Europe/Vienna]', '+01:00[Custom/Vienna]'].forEach((zoneString) =>
+    ['', 'Z', '+01:00[Europe/Vienna]', '+01:00[Custom/Vienna]', '[Europe/Vienna]'].forEach((zoneString) =>
       test(`1976-11-18T15:23:30.123456789${zoneString}[c=iso8601]`, [1976, 11, 18, 15, 23, 30, 123, 456, 789])
     );
   });
@@ -157,7 +158,7 @@ describe('fromString regex', () => {
     // Time separators
     ['T', 't', ' '].forEach((timeSep) => generateTest(`1976-11-18${timeSep}15:23`, ''));
     // Various forms of time zone
-    ['+0100[Europe/Vienna]', '+01:00[Custom/Vienna]', '-0400', ''].forEach((zoneString) =>
+    ['+0100[Europe/Vienna]', '[Europe/Vienna]', '+01:00[Custom/Vienna]', '-0400', ''].forEach((zoneString) =>
       generateTest('1976-11-18T15:23', zoneString)
     );
     // Various numbers of decimal places
@@ -188,7 +189,7 @@ describe('fromString regex', () => {
     test('1512-11-18', [1512, 11, 18]);
     test('15121118', [1512, 11, 18]);
     // Representations with calendar
-    ['', 'Z', '+01:00[Europe/Vienna]', '+01:00[Custom/Vienna]'].forEach((zoneString) =>
+    ['', 'Z', '+01:00[Europe/Vienna]', '[Europe/Vienna]', '+01:00[Custom/Vienna]'].forEach((zoneString) =>
       test(`1976-11-18T15:23:30.123456789${zoneString}[c=iso8601]`, [1976, 11, 18])
     );
     test('1976-11-18[c=iso8601]', [1976, 11, 18]);
@@ -241,11 +242,11 @@ describe('fromString regex', () => {
     test('1976-11-18', []);
     // Time-only forms
     generateTest('15:23', '');
-    ['+01:00[Europe/Vienna]', '+01:00[Custom/Vienna]', '-04:00', 'Z', ''].forEach((zoneStr) =>
+    ['+01:00[Europe/Vienna]', '[Europe/Vienna]', '+01:00[Custom/Vienna]', '-04:00', 'Z', ''].forEach((zoneStr) =>
       test(`15${zoneStr}`, [15])
     );
     // Representations with calendar
-    ['', 'Z', '+01:00[Europe/Vienna]', '+01:00[Custom/Vienna]'].forEach((zoneString) =>
+    ['', 'Z', '+01:00[Europe/Vienna]', '[Europe/Vienna]', '+01:00[Custom/Vienna]'].forEach((zoneString) =>
       test(`1976-11-18T15:23:30.123456789${zoneString}[c=iso8601]`, [15, 23, 30, 123, 456, 789])
     );
     test('15:23:30.123456789[c=iso8601]', [15, 23, 30, 123, 456, 789]);
@@ -270,7 +271,7 @@ describe('fromString regex', () => {
     // Time separators
     ['T', 't', ' '].forEach((timeSep) => generateTest(`1976-11-18${timeSep}15:23`, ''));
     // Various forms of time zone
-    ['+0100[Europe/Vienna]', '+01:00[Custom/Vienna]', '-0400', ''].forEach((zoneString) =>
+    ['+0100[Europe/Vienna]', '[Europe/Vienna]', '+01:00[Custom/Vienna]', '-0400', ''].forEach((zoneString) =>
       generateTest('1976-11-18T15:23', zoneString)
     );
     // Various numbers of decimal places
@@ -310,7 +311,7 @@ describe('fromString regex', () => {
     test('1512-11', [1512, 11]);
     test('151211', [1512, 11]);
     // Representations with calendar
-    ['', 'Z', '+01:00[Europe/Vienna]', '+01:00[Custom/Vienna]'].forEach((zoneString) =>
+    ['', 'Z', '+01:00[Europe/Vienna]', '[Europe/Vienna]', '+01:00[Custom/Vienna]'].forEach((zoneString) =>
       test(`1976-11-18T15:23:30.123456789${zoneString}[c=iso8601]`, [1976, 11])
     );
     test('1976-11-01[c=iso8601]', [1976, 11]);
@@ -335,7 +336,7 @@ describe('fromString regex', () => {
     // Time separators
     ['T', 't', ' '].forEach((timeSep) => generateTest(`1976-11-18${timeSep}15:23`, ''));
     // Various forms of time zone
-    ['+0100[Europe/Vienna]', '+01:00[Custom/Vienna]', '-0400', ''].forEach((zoneString) =>
+    ['+0100[Europe/Vienna]', '[Europe/Vienna]', '+01:00[Custom/Vienna]', '-0400', ''].forEach((zoneString) =>
       generateTest('1976-11-18T15:23', zoneString)
     );
     // Various numbers of decimal places
@@ -378,7 +379,7 @@ describe('fromString regex', () => {
     test('--11-18', [11, 18]);
     test('--1118', [11, 18]);
     // Representations with calendar
-    ['', 'Z', '+01:00[Europe/Vienna]', '+01:00[Custom/Vienna]'].forEach((zoneString) =>
+    ['', 'Z', '+01:00[Europe/Vienna]', '[Europe/Vienna]', '+01:00[Custom/Vienna]'].forEach((zoneString) =>
       test(`1976-11-18T15:23:30.123456789${zoneString}[c=iso8601]`, [11, 18])
     );
     test('1972-11-18[c=iso8601]', [11, 18]);
@@ -424,7 +425,7 @@ describe('fromString regex', () => {
       // Representations with reduced precision
       '1976-11-18T15'
     ].forEach((dateTimeString) => {
-      ['+01:00', '+01', '+0100'].forEach((zoneString) =>
+      ['+01:00', '+01', '+0100', ''].forEach((zoneString) =>
         test(`${dateTimeString}${zoneString}[Europe/Vienna]`, 'Europe/Vienna')
       );
       ['-04:00', '-04', '-0400'].forEach((zoneString) => test(`${dateTimeString}${zoneString}`, '-04:00'));
@@ -452,6 +453,7 @@ describe('fromString regex', () => {
     // Representations with calendar
     test('1976-11-18T15:23:30.123456789Z[c=iso8601]', 'UTC');
     test('1976-11-18T15:23:30.123456789-04:00[c=iso8601]', '-04:00');
+    test('1976-11-18T15:23:30.123456789[Europe/Vienna][c=iso8601]', 'Europe/Vienna');
     test('1976-11-18T15:23:30.123456789+01:00[Europe/Vienna][c=iso8601]', 'Europe/Vienna');
   });
 
diff --git a/polyfill/test/validStrings.mjs b/polyfill/test/validStrings.mjs
index f07f463980..66c68aad6f 100644
--- a/polyfill/test/validStrings.mjs
+++ b/polyfill/test/validStrings.mjs
@@ -216,9 +216,12 @@ const timeZoneIANAName = withCode(
   choice(...timezoneNames),
   (data, result) => (data.ianaName = ES.GetCanonicalTimeZoneIdentifier(result).toString())
 );
-const timeZone = withCode(choice(utcDesignator, seq(timeZoneUTCOffset, ['[', timeZoneIANAName, ']'])), (data) => {
-  if (!('offset' in data)) data.offset = undefined;
-});
+const timeZone = withCode(
+  choice(utcDesignator, timeZoneUTCOffset, seq([timeZoneUTCOffset], '[', timeZoneIANAName, ']')),
+  (data) => {
+    if (!('offset' in data)) data.offset = undefined;
+  }
+);
 const temporalTimeZoneIdentifier = withCode(choice(timeZoneUTCOffset, timeZoneIANAName), (data) => {
   if (!('offset' in data)) data.offset = undefined;
 });
diff --git a/spec/abstractops.html b/spec/abstractops.html
index 0ae886868d..412177a124 100644
--- a/spec/abstractops.html
+++ b/spec/abstractops.html
@@ -640,7 +640,7 @@ <h1>ISO 8601 grammar</h1>
       TimeZone :
           UTCDesignator
           TimeZoneUTCOffset
-          TimeZoneUTCOffset `[` TimeZoneIANAName `]`
+          TimeZoneUTCOffset? `[` TimeZoneIANAName `]`
 
       CalChar :
           Alpha
@@ -1007,8 +1007,9 @@ <h1>ParseTemporalTimeZoneString ( _isoString_ )</h1>
         1. Else,
           1. Set _sign_ to 1.
         1. Set _minute_ to ! ToInteger(_minute_).
-        1. Let _offsetNanoseconds_ be _sign_ × (_hour_ × 60 + _minute_) × 60 × 10<sup>9</sup>.
-        1. Let _name_ be ! FormatTimeZoneOffsetString(_offsetNanoseconds_).
+        1. If _name_ is *undefined*, then
+          1. Let _offsetNanoseconds_ be _sign_ × (_hour_ × 60 + _minute_) × 60 × 10<sup>9</sup>.
+          1. Let _name_ be ! FormatTimeZoneOffsetString(_offsetNanoseconds_).
       1. Assert: ! IsValidTimeZoneName(_name_) is *true*.
       1. Let _name_ be ! CanonicalizeTimeZoneName(_name_).
       1. Return the new Record: {