diff --git a/docs/plaindate.md b/docs/plaindate.md
index b66dbc45a0..8208b77a4a 100644
--- a/docs/plaindate.md
+++ b/docs/plaindate.md
@@ -621,7 +621,7 @@ date.equals(date); // => true
 - `options` (optional object): An object with properties influencing the formatting.
   The following options are recognized:
   - `calendarName` (string): Whether to show the calendar annotation in the return value.
-    Valid values are `'auto'`, `'always'`, and `'never'`.
+    Valid values are `'auto'`, `'always'`, `'never'`, and `'critical'`.
     The default is `'auto'`.
 
 **Returns:** a string in the ISO 8601 date format representing `date`.
@@ -631,6 +631,7 @@ The string can be passed to `Temporal.PlainDate.from()` to create a new `Tempora
 
 Normally, a calendar annotation is shown when `date`'s calendar is not the ISO 8601 calendar.
 By setting the `calendarName` option to `'always'` or `'never'` this can be overridden to always or never show the annotation, respectively.
+Normally not necessary, a value of `'critical'` is equivalent to `'always'` but the annotation will contain an additional `!` for certain interoperation use cases.
 For more information on the calendar annotation, see [the `Temporal` string formats documentation](./strings.md#calendar-systems).
 
 Example usage:
diff --git a/docs/plaindatetime.md b/docs/plaindatetime.md
index d5deea83ef..28855a34aa 100644
--- a/docs/plaindatetime.md
+++ b/docs/plaindatetime.md
@@ -850,7 +850,7 @@ dt1.equals(dt1); // => true
 - `options` (optional object): An object with properties representing options for the operation.
   The following options are recognized:
   - `calendarName` (string): Whether to show the calendar annotation in the return value.
-    Valid values are `'auto'`, `'always'`, and `'never'`.
+    Valid values are `'auto'`, `'always'`, `'never'`, and `'critical'`.
     The default is `'auto'`.
   - `fractionalSecondDigits` (number or string): How many digits to print after the decimal point in the output string.
     Valid values are `'auto'`, 0, 1, 2, 3, 4, 5, 6, 7, 8, or 9.
@@ -875,6 +875,7 @@ Note that rounding may change the value of other units as well.
 
 Normally, a calendar annotation is shown when `datetime`'s calendar is not the ISO 8601 calendar.
 By setting the `calendarName` option to `'always'` or `'never'` this can be overridden to always or never show the annotation, respectively.
+Normally not necessary, a value of `'critical'` is equivalent to `'always'` but the annotation will contain an additional `!` for certain interoperation use cases.
 For more information on the calendar annotation, see [the `Temporal` string formats documentation](./strings.md#calendar-systems).
 
 Example usage:
diff --git a/docs/plainmonthday.md b/docs/plainmonthday.md
index 238e8fff85..f5992b22fc 100644
--- a/docs/plainmonthday.md
+++ b/docs/plainmonthday.md
@@ -247,7 +247,7 @@ md2.equals({ monthCode: 'M02', day: 29 }); // => true
 - `options` (optional object): An object with properties influencing the formatting.
   The following options are recognized:
   - `calendarName` (string): Whether to show the calendar annotation in the return value.
-    Valid values are `'auto'`, `'always'`, and `'never'`.
+    Valid values are `'auto'`, `'always'`, `'never'`, and `'critical'`.
     The default is `'auto'`.
 
 **Returns:** a string in the ISO 8601 date format representing `monthDay`.
@@ -257,6 +257,7 @@ The string can be passed to `Temporal.PlainMonthDay.from()` to create a new `Tem
 
 Normally, a calendar annotation is shown when `monthDay`'s calendar is not the ISO 8601 calendar.
 By setting the `calendarName` option to `'always'` or `'never'` this can be overridden to always or never show the annotation, respectively.
+Normally not necessary, a value of `'critical'` is equivalent to `'always'` but the annotation will contain an additional `!` for certain interoperation use cases.
 For more information on the calendar annotation, see [the `Temporal` string formats documentation](./strings.md#calendar-systems).
 
 Example usage:
diff --git a/docs/plainyearmonth.md b/docs/plainyearmonth.md
index 29a5b7dd8f..10b34d11c1 100644
--- a/docs/plainyearmonth.md
+++ b/docs/plainyearmonth.md
@@ -509,7 +509,7 @@ ym.equals(ym); // => true
 - `options` (optional object): An object with properties influencing the formatting.
   The following options are recognized:
   - `calendarName` (string): Whether to show the calendar annotation in the return value.
-    Valid values are `'auto'`, `'always'`, and `'never'`.
+    Valid values are `'auto'`, `'always'`, `'never'`, and `'critical'`.
     The default is `'auto'`.
 
 **Returns:** a string in the ISO 8601 date format representing `yearMonth`.
@@ -519,6 +519,7 @@ The string can be passed to `Temporal.PlainYearMonth.from()` to create a new `Te
 
 Normally, a calendar annotation is shown when `yearMonth`'s calendar is not the ISO 8601 calendar.
 By setting the `calendarName` option to `'always'` or `'never'` this can be overridden to always or never show the annotation, respectively.
+Normally not necessary, a value of `'critical'` is equivalent to `'always'` but the annotation will contain an additional `!` for certain interoperation use cases.
 For more information on the calendar annotation, see [the `Temporal` string formats documentation](./strings.md#calendar-systems).
 
 Example usage:
diff --git a/docs/zoneddatetime.md b/docs/zoneddatetime.md
index d94f250549..5744c02499 100644
--- a/docs/zoneddatetime.md
+++ b/docs/zoneddatetime.md
@@ -1248,7 +1248,7 @@ zdt1.equals(zdt1); // => true
     Valid values are `'auto'` and `'never'`.
     The default is `'auto'`.
   - `calendarName` (string): Whether to show the calendar annotation in the return value.
-    Valid values are `'auto'`, `'always'`, and `'never'`.
+    Valid values are `'auto'`, `'always'`, `'never'`, and `'critical'`.
     The default is `'auto'`.
   - `fractionalSecondDigits` (number or string): How many digits to print after the decimal point in the output string.
     Valid values are `'auto'`, 0, 1, 2, 3, 4, 5, 6, 7, 8, or 9.
@@ -1279,6 +1279,7 @@ Note that rounding may change the value of other units as well.
 
 Normally, a calendar annotation is shown when `zonedDateTime`'s calendar is not the ISO 8601 calendar.
 By setting the `calendarName` option to `'always'` or `'never'` this can be overridden to always or never show the annotation, respectively.
+Normally not necessary, a value of `'critical'` is equivalent to `'always'` but the annotation will contain an additional `!` for certain interoperation use cases.
 For more information on the calendar annotation, see [ISO string extensions](./strings.md#calendar-systems).
 
 Likewise, passing `'never'` to the `timeZoneName` or `offset` options controls whether the time zone offset (`+01:00`) or name annotation (`[Europe/Paris]`) are shown.
diff --git a/polyfill/lib/ecmascript.mjs b/polyfill/lib/ecmascript.mjs
index 248f841d7e..eba28fac11 100644
--- a/polyfill/lib/ecmascript.mjs
+++ b/polyfill/lib/ecmascript.mjs
@@ -286,7 +286,8 @@ export const ES = ObjectAssign({}, ES2020, {
   FormatCalendarAnnotation: (id, showCalendar) => {
     if (showCalendar === 'never') return '';
     if (showCalendar === 'auto' && id === 'iso8601') return '';
-    return `[u-ca=${id}]`;
+    const flag = showCalendar === 'critical' ? '!' : '';
+    return `[${flag}u-ca=${id}]`;
   },
   ParseISODateTime: (isoString) => {
     // ZDT is the superset of fields for every other Temporal type
@@ -718,7 +719,7 @@ export const ES = ObjectAssign({}, ES2020, {
     return ES.GetOption(options, 'offset', ['prefer', 'use', 'ignore', 'reject'], fallback);
   },
   ToShowCalendarOption: (options) => {
-    return ES.GetOption(options, 'calendarName', ['auto', 'always', 'never'], 'auto');
+    return ES.GetOption(options, 'calendarName', ['auto', 'always', 'never', 'critical'], 'auto');
   },
   ToShowTimeZoneNameOption: (options) => {
     return ES.GetOption(options, 'timeZoneName', ['auto', 'never'], 'auto');
@@ -2085,7 +2086,7 @@ export const ES = ObjectAssign({}, ES2020, {
     let resultString = `${month}-${day}`;
     const calendar = GetSlot(monthDay, CALENDAR);
     const calendarID = ES.ToString(calendar);
-    if (showCalendar === 'always' || calendarID !== 'iso8601') {
+    if (showCalendar === 'always' || showCalendar === 'critical' || calendarID !== 'iso8601') {
       const year = ES.ISOYearString(GetSlot(monthDay, ISO_YEAR));
       resultString = `${year}-${resultString}`;
     }
@@ -2099,7 +2100,7 @@ export const ES = ObjectAssign({}, ES2020, {
     let resultString = `${year}-${month}`;
     const calendar = GetSlot(yearMonth, CALENDAR);
     const calendarID = ES.ToString(calendar);
-    if (showCalendar === 'always' || calendarID !== 'iso8601') {
+    if (showCalendar === 'always' || showCalendar === 'critical' || calendarID !== 'iso8601') {
       const day = ES.ISODateTimePartString(GetSlot(yearMonth, ISO_DAY));
       resultString += `-${day}`;
     }
diff --git a/spec/abstractops.html b/spec/abstractops.html
index 3e8e5c771b..5d8fc37593 100644
--- a/spec/abstractops.html
+++ b/spec/abstractops.html
@@ -148,7 +148,7 @@ <h1>ToShowCalendarOption ( _normalizedOptions_ )</h1>
       This property is used in `toString` methods in Temporal to control whether a calendar annotation should be output.
     </emu-note>
     <emu-alg>
-      1. Return ? GetOption(_normalizedOptions_, *"calendarName"*, *"string"*, « *"auto"*, *"always"*, *"never"* », *"auto"*).
+      1. Return ? GetOption(_normalizedOptions_, *"calendarName"*, *"string"*, « *"auto"*, *"always"*, *"never"*, *"critical"* », *"auto"*).
     </emu-alg>
   </emu-clause>
 
diff --git a/spec/calendar.html b/spec/calendar.html
index b16c9fc8e7..dfb2df5f14 100644
--- a/spec/calendar.html
+++ b/spec/calendar.html
@@ -393,7 +393,7 @@ <h1>
       <h1>
         MaybeFormatCalendarAnnotation (
           _calendarObject_: an Object or *undefined*,
-          _showCalendar_: one of *"auto"*, *"always"*, or *"never"*,
+          _showCalendar_: one of *"auto"*, *"always"*, *"never"*, or *"critical"*,
         ): either a normal completion containing a String, or an abrupt completion
       </h1>
       <dl class="header">
@@ -419,7 +419,7 @@ <h1>
       <h1>
         FormatCalendarAnnotation (
           _id_: a String,
-          _showCalendar_: one of *"auto"*, *"always"*, or *"never"*,
+          _showCalendar_: one of *"auto"*, *"always"*, *"never"*, or *"critical"*,
         ): a String
       </h1>
       <dl class="header">
@@ -432,7 +432,8 @@ <h1>
       <emu-alg>
         1. If _showCalendar_ is *"never"*, return the empty String.
         1. If _showCalendar_ is *"auto"* and _id_ is *"iso8601"*, return the empty String.
-        1. Return the string-concatenation of *"[u-ca="*, _id_, and *"]"*.
+        1. If _showCalendar_ is *"critical"*, let _flag_ be *"!"*; else, let flag be the empty String.
+        1. Return the string-concatenation of *"["*, _flag_, *"u-ca="*, _id_, and *"]"*.
       </emu-alg>
       <emu-note type="editor">
         The exact form this annotation will take is undergoing a standardization process in the IETF; it is being discussed in the Internet Draft <a href="https://datatracker.ietf.org/doc/html/draft-ietf-sedate-datetime-extended">Date and Time on the Internet: Timestamps with additional information</a>.
diff --git a/spec/plainmonthday.html b/spec/plainmonthday.html
index 72fa5ce3eb..d86a1f5ad8 100644
--- a/spec/plainmonthday.html
+++ b/spec/plainmonthday.html
@@ -408,7 +408,7 @@ <h1>TemporalMonthDayToString ( _monthDay_, _showCalendar_ )</h1>
         1. Let _day_ be ToZeroPaddedDecimalString(_monthDay_.[[ISODay]], 2).
         1. Let _result_ be the string-concatenation of _month_, the code unit 0x002D (HYPHEN-MINUS), and _day_.
         1. Let _calendarID_ be ? ToString(_monthDay_.[[Calendar]]).
-        1. If _showCalendar_ is *"always"* or if _calendarID_ is not *"iso8601"*, then
+        1. If _showCalendar_ is one of *"always"* or *"critical"*, or if _calendarID_ is not *"iso8601"*, then
           1. Let _year_ be ! PadISOYear(_monthDay_.[[ISOYear]]).
           1. Set _result_ to the string-concatenation of _year_, the code unit 0x002D (HYPHEN-MINUS), and _result_.
         1. Let _calendarString_ be FormatCalendarAnnotation(_calendarID_, _showCalendar_).
diff --git a/spec/plainyearmonth.html b/spec/plainyearmonth.html
index d438371f95..c684693e45 100644
--- a/spec/plainyearmonth.html
+++ b/spec/plainyearmonth.html
@@ -587,7 +587,7 @@ <h1>TemporalYearMonthToString ( _yearMonth_, _showCalendar_ )</h1>
         1. Let _month_ be ToZeroPaddedDecimalString(_yearMonth_.[[ISOMonth]], 2).
         1. Let _result_ be the string-concatenation of _year_, the code unit 0x002D (HYPHEN-MINUS), and _month_.
         1. Let _calendarID_ be ? ToString(_yearMonth_.[[Calendar]]).
-        1. If _showCalendar_ is *"always"* or if _calendarID_ is not *"iso8601"*, then
+        1. If _showCalendar_ is one of *"always"* or *"critical"*, or if _calendarID_ is not *"iso8601"*, then
           1. Let _day_ be ToZeroPaddedDecimalString(_yearMonth_.[[ISODay]], 2).
           1. Set _result_ to the string-concatenation of _result_, the code unit 0x002D (HYPHEN-MINUS), and _day_.
         1. Let _calendarString_ be FormatCalendarAnnotation(_calendarID_, _showCalendar_).
diff --git a/spec/zoneddatetime.html b/spec/zoneddatetime.html
index 40dec73716..b3fa2fd0e3 100644
--- a/spec/zoneddatetime.html
+++ b/spec/zoneddatetime.html
@@ -1162,7 +1162,7 @@ <h1>
         TemporalZonedDateTimeToString (
           _zonedDateTime_: a Temporal.ZonedDateTime,
           _precision_: one of *"auto"*, *"minute"*, or an integer between 0 and 9 inclusive,
-          _showCalendar_: one of *"auto"*, *"always"*, or *"never"*,
+          _showCalendar_: one of *"auto"*, *"always"*, *"never"*, or *"critical"*,
           _showTimeZone_: one of *"auto"* or *"never"*,
           _showOffset_: one of *"auto"* or *"never"*,
           optional _increment_: an integer,