diff --git a/polyfill/lib/duration.mjs b/polyfill/lib/duration.mjs
index 7c1e62271f..42b0f4ce3e 100644
--- a/polyfill/lib/duration.mjs
+++ b/polyfill/lib/duration.mjs
@@ -339,7 +339,14 @@ export class Duration {
       ) {
         calendarRec.lookup('dateAdd');
       }
-      if (largestUnit === 'year' || largestUnit === 'month' || largestUnit === 'week' || smallestUnit === 'year') {
+      if (
+        largestUnit === 'year' ||
+        largestUnit === 'month' ||
+        largestUnit === 'week' ||
+        smallestUnit === 'year' ||
+        smallestUnit === 'month' ||
+        smallestUnit === 'week'
+      ) {
         calendarRec.lookup('dateUntil');
       }
     }
@@ -479,7 +486,7 @@ export class Duration {
       if (years !== 0 || months !== 0 || weeks !== 0 || unit === 'year' || unit === 'month' || unit === 'week') {
         calendarRec.lookup('dateAdd');
       }
-      if (unit === 'year' || (unit === 'month' && years !== 0)) {
+      if (unit === 'year' || unit === 'month' || unit === 'week') {
         calendarRec.lookup('dateUntil');
       }
     }
diff --git a/polyfill/lib/ecmascript.mjs b/polyfill/lib/ecmascript.mjs
index 32286c115d..f8d03ef257 100644
--- a/polyfill/lib/ecmascript.mjs
+++ b/polyfill/lib/ecmascript.mjs
@@ -4143,7 +4143,9 @@ export function DifferenceTemporalPlainDate(operation, plainDate, other, options
     settings.largestUnit === 'year' ||
     settings.largestUnit === 'month' ||
     settings.largestUnit === 'week' ||
-    settings.smallestUnit === 'year'
+    settings.smallestUnit === 'year' ||
+    settings.smallestUnit === 'month' ||
+    settings.smallestUnit === 'week'
   ) {
     calendarRec.lookup('dateUntil');
   }
@@ -4211,7 +4213,9 @@ export function DifferenceTemporalPlainDateTime(operation, plainDateTime, other,
   if (
     (!datePartsIdentical &&
       (settings.largestUnit === 'year' || settings.largestUnit === 'month' || settings.largestUnit === 'week')) ||
-    settings.smallestUnit === 'year'
+    settings.smallestUnit === 'year' ||
+    settings.smallestUnit === 'month' ||
+    settings.smallestUnit === 'week'
   ) {
     calendarRec.lookup('dateUntil');
   }
@@ -5498,8 +5502,11 @@ export function RoundDuration(
   zonedRelativeTo = undefined,
   timeZoneRec = undefined
 ) {
-  // dateAdd, dateUntil must be looked up if smallestUnit == year
-  // dateAdd must be looked up if smallestUnit == month or week
+  // calendarRec must have looked up:
+  // dateAdd, if
+  //  smallestUnit == year, month, week or
+  //  (smallestUnit == day && any of years, months, weeks days ≠ 0 && zonedRelativeTo defined)
+  // dateUntil, if smallestUnit == year, month, week && any of years, months, weeks ≠ 0
   const TemporalDuration = GetIntrinsic('%Temporal.Duration%');
 
   if ((unit === 'year' || unit === 'month' || unit === 'week') && !plainRelativeTo) {
@@ -5598,21 +5605,32 @@ export function RoundDuration(
       plainRelativeTo = yearsMonthsLater;
       days += weeksInDays;
 
-      // Months may be different lengths of days depending on the calendar,
-      // convert days to months in a loop as described above under 'years'.
-      const sign = MathSign(days);
+      const isoResult = AddISODate(
+        GetSlot(plainRelativeTo, ISO_YEAR),
+        GetSlot(plainRelativeTo, ISO_MONTH),
+        GetSlot(plainRelativeTo, ISO_DAY),
+        0,
+        0,
+        0,
+        days,
+        'constrain'
+      );
+      const wholeDaysLater = CreateTemporalDate(isoResult.year, isoResult.month, isoResult.day, calendarRec.receiver);
+      const untilOptions = ObjectCreate(null);
+      untilOptions.largestUnit = 'month';
+      const monthsPassed = GetSlot(DifferenceDate(calendarRec, plainRelativeTo, wholeDaysLater, untilOptions), MONTHS);
+      months += monthsPassed;
+      const monthsPassedDuration = new TemporalDuration(0, monthsPassed);
+      let daysPassed;
+      ({ relativeTo: plainRelativeTo, days: daysPassed } = MoveRelativeDate(
+        calendarRec,
+        plainRelativeTo,
+        monthsPassedDuration
+      ));
+      days -= daysPassed;
       const oneMonth = new TemporalDuration(0, days < 0 ? -1 : 1);
-      let oneMonthDays;
-      ({ relativeTo: plainRelativeTo, days: oneMonthDays } = MoveRelativeDate(calendarRec, plainRelativeTo, oneMonth));
-      while (MathAbs(days) >= MathAbs(oneMonthDays)) {
-        months += sign;
-        days -= oneMonthDays;
-        ({ relativeTo: plainRelativeTo, days: oneMonthDays } = MoveRelativeDate(
-          calendarRec,
-          plainRelativeTo,
-          oneMonth
-        ));
-      }
+      let { days: oneMonthDays } = MoveRelativeDate(calendarRec, plainRelativeTo, oneMonth);
+
       oneMonthDays = MathAbs(oneMonthDays);
       const divisor = bigInt(oneMonthDays).multiply(dayLengthNs);
       nanoseconds = divisor.multiply(months).plus(bigInt(days).multiply(dayLengthNs)).plus(nanoseconds);
@@ -5624,17 +5642,32 @@ export function RoundDuration(
       break;
     }
     case 'week': {
-      // Weeks may be different lengths of days depending on the calendar,
-      // convert days to weeks in a loop as described above under 'years'.
-      const sign = MathSign(days);
+      const isoResult = AddISODate(
+        GetSlot(plainRelativeTo, ISO_YEAR),
+        GetSlot(plainRelativeTo, ISO_MONTH),
+        GetSlot(plainRelativeTo, ISO_DAY),
+        0,
+        0,
+        0,
+        days,
+        'constrain'
+      );
+      const wholeDaysLater = CreateTemporalDate(isoResult.year, isoResult.month, isoResult.day, calendarRec.receiver);
+      const untilOptions = ObjectCreate(null);
+      untilOptions.largestUnit = 'week';
+      const weeksPassed = GetSlot(DifferenceDate(calendarRec, plainRelativeTo, wholeDaysLater, untilOptions), WEEKS);
+      weeks += weeksPassed;
+      const weeksPassedDuration = new TemporalDuration(0, 0, weeksPassed);
+      let daysPassed;
+      ({ relativeTo: plainRelativeTo, days: daysPassed } = MoveRelativeDate(
+        calendarRec,
+        plainRelativeTo,
+        weeksPassedDuration
+      ));
+      days -= daysPassed;
       const oneWeek = new TemporalDuration(0, 0, days < 0 ? -1 : 1);
-      let oneWeekDays;
-      ({ relativeTo: plainRelativeTo, days: oneWeekDays } = MoveRelativeDate(calendarRec, plainRelativeTo, oneWeek));
-      while (MathAbs(days) >= MathAbs(oneWeekDays)) {
-        weeks += sign;
-        days -= oneWeekDays;
-        ({ relativeTo: plainRelativeTo, days: oneWeekDays } = MoveRelativeDate(calendarRec, plainRelativeTo, oneWeek));
-      }
+      let { days: oneWeekDays } = MoveRelativeDate(calendarRec, plainRelativeTo, oneWeek);
+
       oneWeekDays = MathAbs(oneWeekDays);
       const divisor = bigInt(oneWeekDays).multiply(dayLengthNs);
       nanoseconds = divisor.multiply(weeks).plus(bigInt(days).multiply(dayLengthNs)).plus(nanoseconds);
diff --git a/spec/duration.html b/spec/duration.html
index 9fdb8763e3..84e9dd464f 100644
--- a/spec/duration.html
+++ b/spec/duration.html
@@ -478,7 +478,7 @@ <h1>Temporal.Duration.prototype.round ( _roundTo_ )</h1>
           1. Set _calendarRec_ to ! CreateCalendarRecord(_calendar_, « »).
           1. If _duration_.[[Years]] &ne; 0; or _duration_.[[Months]] &ne; 0; or _duration_.[[Weeks]] &ne; 0; or _duration_.[[Days]] &ne; 0 and _largestUnit_ is any of *"year"*, *"month"*, *"week"*; or _smallestUnit_ is any of *"year"*, *"month"*, *"week"*; then
             1. Set _calendarRec_.[[DateAdd]] to ? GetMethod(_calendar_, *"dateAdd"*).
-          1. If _largestUnit_ is any of *"year"*, *"month"*, *"week"*, or _smallestUnit_ is *"year"*, then
+          1. If _largestUnit_ is any of *"year"*, *"month"*, *"week"*, or _smallestUnit_ is any of *"year"*, *"month"*, *"week"*, then
             1. Set _calendarRec_.[[DateUntil]] to ? GetMethod(_calendar_, *"dateUntil"*).
         1. Let _unbalanceResult_ be ? UnbalanceDateDurationRelative(_duration_.[[Years]], _duration_.[[Months]], _duration_.[[Weeks]], _duration_.[[Days]], _largestUnit_, _plainRelativeTo_, _calendarRec_).
         1. Let _roundRecord_ be ? RoundDuration(_unbalanceResult_.[[Years]], _unbalanceResult_.[[Months]], _unbalanceResult_.[[Weeks]], _unbalanceResult_.[[Days]], _duration_.[[Hours]], _duration_.[[Minutes]], _duration_.[[Seconds]], _duration_.[[Milliseconds]], _duration_.[[Microseconds]], _duration_.[[Nanoseconds]], _roundingIncrement_, _smallestUnit_, _roundingMode_, _plainRelativeTo_, _calendarRec_, _zonedRelativeTo_, _timeZoneRec_).
@@ -532,7 +532,7 @@ <h1>Temporal.Duration.prototype.total ( _totalOf_ )</h1>
           1. Set _calendarRec_ to ! CreateCalendarRecord(_calendar_, « »).
           1. If _duration_.[[Years]] &ne; 0; or _duration_.[[Months]] &ne; 0; or _duration_.[[Weeks]] &ne; 0; or _unit_ is any of *"year"*, *"month"*, *"week"*; then
             1. Set _calendarRec_.[[DateAdd]] to ? GetMethod(_calendar_, *"dateAdd"*).
-          1. If _unit_ is *"year"*, or _unit_ is *"month"* and _duration_.[[Years]] &ne; 0, then
+          1. If _unit_ is any of *"year"*, *"month"*, *"week"*, then
             1. Set _calendarRec_.[[DateUntil]] to ? GetMethod(_calendar_, *"dateUntil"*).
         1. Let _unbalanceResult_ be ? UnbalanceDateDurationRelative(_duration_.[[Years]], _duration_.[[Months]], _duration_.[[Weeks]], _duration_.[[Days]], _unit_, _plainRelativeTo_, _calendarRec_).
         1. If _zonedRelativeTo_ is not *undefined*, then
@@ -1656,10 +1656,12 @@ <h1>
       <emu-alg>
         1. Assert: If either of _plainRelativeTo_ or _zonedRelativeTo_ are present and not *undefined*, _calendarRec_ is not *undefined*.
         1. Assert: If _zonedRelativeTo_ is present and not *undefined*, _timeZoneRec_ is not *undefined*.
-        1. Assert: If _unit_ is *"year"*, neither of _calendarRec_.[[DateAdd]] or _calendarRec_.[[DateUntil]] are *undefined*.
-        1. Assert: If _unit_ is *"month"* or *"week"*, _calendarRec_.[[DateAdd]] is not *undefined*.
+        1. If _unit_ is any of *"year"*, *"month"*, *"week"*, then
+          1. Assert: _calendarRec_.[[DateAdd]] is not *undefined*.
+          1. Assert: If _years_ &ne; 0, or _months_ &ne; 0, or _weeks_ &ne; 0, _calendarRec_.[[DateUntil]] is not *undefined*.
         1. If _plainRelativeTo_ is not present, set _plainRelativeTo_ to *undefined*.
         1. If _zonedRelativeTo_ is not present, set _zonedRelativeTo_ to *undefined*.
+        1. Assert: If _unit_ is *"day"*; and _zonedRelativeTo_ is not *undefined*; and _years_ &ne; 0, or _months_ &ne; 0, or _weeks_ &ne; 0, or _days_ &ne; 0, _calendarRec_.[[DateAdd]] is not *undefined*.
         1. If _unit_ is *"year"*, *"month"*, or *"week"*, and _plainRelativeTo_ is *undefined*, then
           1. Throw a *RangeError* exception.
         1. If _unit_ is one of *"year"*, *"month"*, *"week"*, or *"day"*, then
@@ -1712,33 +1714,43 @@ <h1>
           1. Let _weeksInDays_ be DaysUntil(_yearsMonthsLater_, _yearsMonthsWeeksLater_).
           1. Set _plainRelativeTo_ to _yearsMonthsLater_.
           1. Set _fractionalDays_ to _fractionalDays_ + _weeksInDays_.
+          1. Let _isoResult_ be ! AddISODate(_plainRelativeTo_.[[ISOYear]], _plainRelativeTo_.[[ISOMonth]], _plainRelativeTo_.[[ISODay]], 0, 0, 0, truncate(_fractionalDays_), *"constrain"*).
+          1. Let _wholeDaysLater_ be ! CreateTemporalDate(_isoResult_.[[Year]], _isoResult_.[[Month]], _isoResult_.[[Day]], _calendarRec_.[[Receiver]]).
+          1. Let _untilOptions_ be OrdinaryObjectCreate(*null*).
+          1. Perform ! CreateDataPropertyOrThrow(_untilOptions_, *"largestUnit"*, *"month"*).
+          1. Let _timePassed_ be ? DifferenceDate(_calendarRec_, _plainRelativeTo_, _wholeDaysLater_, _untilOptions_).
+          1. Let _monthsPassed_ be _timePassed_.[[Months]].
+          1. Set _months_ to _months_ + _monthsPassed_.
+          1. Let _monthsPassedDuration_ be ! CreateTemporalDuration(0, _monthsPassed_, 0, 0, 0, 0, 0, 0, 0, 0).
+          1. Let _moveResult_ be ? MoveRelativeDate(_calendarRec_, _plainRelativeTo_, _monthsPassedDuration_).
+          1. Set _plainRelativeTo_ to _moveResult_.[[RelativeTo]].
+          1. Let _daysPassed_ be _moveResult_.[[Days]].
+          1. Set _fractionalDays_ to _fractionalDays_ - _daysPassed_.
           1. If _fractionalDays_ &lt; 0, let _sign_ be -1; else, let _sign_ be 1.
           1. Let _oneMonth_ be ! CreateTemporalDuration(0, _sign_, 0, 0, 0, 0, 0, 0, 0, 0).
           1. Let _moveResult_ be ? MoveRelativeDate(_calendarRec_, _plainRelativeTo_, _oneMonth_).
-          1. Set _plainRelativeTo_ to _moveResult_.[[RelativeTo]].
           1. Let _oneMonthDays_ be _moveResult_.[[Days]].
-          1. Repeat, while abs(_fractionalDays_) &ge; abs(_oneMonthDays_),
-            1. Set _months_ to _months_ + _sign_.
-            1. Set _fractionalDays_ to _fractionalDays_ - _oneMonthDays_.
-            1. Set _moveResult_ to ? MoveRelativeDate(_calendarRec_, _plainRelativeTo_, _oneMonth_).
-            1. Set _plainRelativeTo_ to _moveResult_.[[RelativeTo]].
-            1. Set _oneMonthDays_ to _moveResult_.[[Days]].
           1. Let _fractionalMonths_ be _months_ + _fractionalDays_ / abs(_oneMonthDays_).
           1. Set _months_ to RoundNumberToIncrement(_fractionalMonths_, _increment_, _roundingMode_).
           1. Set _total_ to _fractionalMonths_.
           1. Set _weeks_ to 0.
         1. Else if _unit_ is *"week"*, then
+          1. Let _isoResult_ be ! AddISODate(_plainRelativeTo_.[[ISOYear]], _plainRelativeTo_.[[ISOMonth]], _plainRelativeTo_.[[ISODay]], 0, 0, 0, truncate(_fractionalDays_), *"constrain"*).
+          1. Let _wholeDaysLater_ be ! CreateTemporalDate(_isoResult_.[[Year]], _isoResult_.[[Month]], _isoResult_.[[Day]], _calendarRec_.[[Receiver]]).
+          1. Let _untilOptions_ be OrdinaryObjectCreate(*null*).
+          1. Perform ! CreateDataPropertyOrThrow(_untilOptions_, *"largestUnit"*, *"week"*).
+          1. Let _timePassed_ be ? DifferenceDate(_calendarRec_, _plainRelativeTo_, _wholeDaysLater_, _untilOptions_).
+          1. Let _weeksPassed_ be _timePassed_.[[Weeks]].
+          1. Set _weeks_ to _weeks_ + _weeksPassed_.
+          1. Let _weeksPassedDuration_ be ! CreateTemporalDuration(0, 0, _weeksPassed_, 0, 0, 0, 0, 0, 0, 0).
+          1. Let _moveResult_ be ? MoveRelativeDate(_calendarRec_, _plainRelativeTo_, _weeksPassedDuration_).
+          1. Set _plainRelativeTo_ to _moveResult_.[[RelativeTo]].
+          1. Let _daysPassed_ be _moveResult_.[[Days]].
+          1. Set _fractionalDays_ to _fractionalDays_ - _daysPassed_.
           1. If _fractionalDays_ &lt; 0, let _sign_ be -1; else, let _sign_ be 1.
           1. Let _oneWeek_ be ! CreateTemporalDuration(0, 0, _sign_, 0, 0, 0, 0, 0, 0, 0).
           1. Let _moveResult_ be ? MoveRelativeDate(_calendarRec_, _plainRelativeTo_, _oneWeek_).
-          1. Set _plainRelativeTo_ to _moveResult_.[[RelativeTo]].
           1. Let _oneWeekDays_ be _moveResult_.[[Days]].
-          1. Repeat, while abs(_fractionalDays_) &ge; abs(_oneWeekDays_),
-            1. Set _weeks_ to _weeks_ + _sign_.
-            1. Set _fractionalDays_ to _fractionalDays_ - _oneWeekDays_.
-            1. Set _moveResult_ to ? MoveRelativeDate(_calendarRec_, _plainRelativeTo_, _oneWeek_).
-            1. Set _plainRelativeTo_ to _moveResult_.[[RelativeTo]].
-            1. Set _oneWeekDays_ to _moveResult_.[[Days]].
           1. Let _fractionalWeeks_ be _weeks_ + _fractionalDays_ / abs(_oneWeekDays_).
           1. Set _weeks_ to RoundNumberToIncrement(_fractionalWeeks_, _increment_, _roundingMode_).
           1. Set _total_ to _fractionalWeeks_.
diff --git a/spec/plaindate.html b/spec/plaindate.html
index 025e16122b..6db2020f5b 100644
--- a/spec/plaindate.html
+++ b/spec/plaindate.html
@@ -1055,7 +1055,7 @@ <h1>
         1. Let _calendarRec_ be ! CreateCalendarRecord(_temporalDate_.[[Calendar]], « »).
         1. If _settings_.[[SmallestUnit]] is one of *"year"*, *"month"*, or *"week"*, then
           1. Set _calendarRec_.[[DateAdd]] to ? GetMethod(_temporalDate_.[[Calendar]], *"dateAdd"*).
-        1. If _settings_.[[LargestUnit]] is one of *"year"*, *"month"*, or *"week"*, or _settings_.[[SmallestUnit]] is *"year"*, then
+        1. If _settings_.[[LargestUnit]] is one of *"year"*, *"month"*, or *"week"*, or _settings_.[[SmallestUnit]] is one of *"year"*, *"month"*, *"week"*, then
           1. Set _calendarRec_.[[DateUntil]] to ? GetMethod(_temporalDate_.[[Calendar]], *"dateUntil"*).
         1. Let _result_ be ? DifferenceDate(_calendarRec_, _temporalDate_, _other_, _resolvedOptions_).
         1. If _settings_.[[SmallestUnit]] is not *"day"* or _settings_.[[RoundingIncrement]] &ne; 1, then
diff --git a/spec/plaindatetime.html b/spec/plaindatetime.html
index b45e0321e3..8b61bcb9ba 100644
--- a/spec/plaindatetime.html
+++ b/spec/plaindatetime.html
@@ -1194,7 +1194,7 @@ <h1>
         1. Let _largestUnitRequiresDateUntilLookup_ be *false*.
         1. If _datePartsIdentical_ is *false* and _settings_.[[LargestUnit]] is one of *"year"*, *"month"*, or *"week"*, then
           1. Set _largestUnitRequiresDateUntilLookup_ to *true*.
-        1. If _largestUnitRequiresDateUntilLookup_ is *true*, or _settings_.[[SmallestUnit]] is *"year"*, then
+        1. If _largestUnitRequiresDateUntilLookup_ is *true*, or _settings_.[[SmallestUnit]] is one of *"year"*, *"month"*, *"week"*, then
           1. Set _calendarRec_.[[DateUntil]] to ? GetMethod(_dateTime_.[[Calendar]], *"dateUntil"*).
         1. Let _diff_ be ? DifferenceISODateTime(_dateTime_.[[ISOYear]], _dateTime_.[[ISOMonth]], _dateTime_.[[ISODay]], _dateTime_.[[ISOHour]], _dateTime_.[[ISOMinute]], _dateTime_.[[ISOSecond]], _dateTime_.[[ISOMillisecond]], _dateTime_.[[ISOMicrosecond]], _dateTime_.[[ISONanosecond]], _other_.[[ISOYear]], _other_.[[ISOMonth]], _other_.[[ISODay]], _other_.[[ISOHour]], _other_.[[ISOMinute]], _other_.[[ISOSecond]], _other_.[[ISOMillisecond]], _other_.[[ISOMicrosecond]], _other_.[[ISONanosecond]], _dateTime_.[[Calendar]], _settings_.[[LargestUnit]], _resolvedOptions_, _calendarRec_).
         1. If _settings_.[[SmallestUnit]] is *"nanosecond"* and _settings_.[[RoundingIncrement]] is 1, then