Normative: Don't convert relativeTo to PlainDate twice in Duration.co…

…mpare

We call UnbalanceDurationRelative twice with the same relativeTo object.
UnbalanceDurationRelative only uses plain or undefined relativeTo, not
zoned, so it will convert it with ToTemporalDate. No need to do this
twice; pre-convert it before the first call.

See: #2247

polyfill/lib/duration.mjs

@@ -549,7 +549,7 @@ export class Duration {
     one = ES.ToTemporalDuration(one);
     two = ES.ToTemporalDuration(two);
     options = ES.GetOptionsObject(options);
-    const relativeTo = ES.ToRelativeTemporalObject(options);
+    let relativeTo = ES.ToRelativeTemporalObject(options);
     const y1 = GetSlot(one, YEARS);
     const mon1 = GetSlot(one, MONTHS);
     const w1 = GetSlot(one, WEEKS);
@@ -573,6 +573,7 @@ export class Duration {
     const shift1 = ES.CalculateOffsetShift(relativeTo, y1, mon1, w1, d1);
     const shift2 = ES.CalculateOffsetShift(relativeTo, y2, mon2, w2, d2);
     if (y1 !== 0 || y2 !== 0 || mon1 !== 0 || mon2 !== 0 || w1 !== 0 || w2 !== 0) {
+      if (ES.IsTemporalZonedDateTime(relativeTo)) relativeTo = ES.ToTemporalDate(relativeTo);
       ({ days: d1 } = ES.UnbalanceDateDurationRelative(y1, mon1, w1, d1, 'day', relativeTo));
       ({ days: d2 } = ES.UnbalanceDateDurationRelative(y2, mon2, w2, d2, 'day', relativeTo));
     }

spec/duration.html

@@ -98,6 +98,7 @@ <h1>Temporal.Duration.compare ( _one_, _two_ [ , _options_ ] )</h1>
         1. Let _shift1_ be ? CalculateOffsetShift(_relativeTo_, _one_.[[Years]], _one_.[[Months]], _one_.[[Weeks]], _one_.[[Days]]).
         1. Let _shift2_ be ? CalculateOffsetShift(_relativeTo_, _two_.[[Years]], _two_.[[Months]], _two_.[[Weeks]], _two_.[[Days]]).
         1. If _one_.[[Years]] &ne; 0, or _two_.[[Years]] &ne; 0, or _one_.[[Months]] &ne; 0, or _two_.[[Months]] &ne; 0, or _one_.[[Weeks]] &ne; 0, or _two_.[[Weeks]] &ne; 0, then
+          1. If _relativeTo_ is an Object with an [[InitializedTemporalZonedDateTime]] internal slot, set _relativeTo_ to ? ToTemporalDate(_relativeTo_).
           1. Let _unbalanceResult1_ be ? UnbalanceDateDurationRelative(_one_.[[Years]], _one_.[[Months]], _one_.[[Weeks]], _one_.[[Days]], *"day"*, _relativeTo_).
           1. Let _unbalanceResult2_ be ? UnbalanceDateDurationRelative(_two_.[[Years]], _two_.[[Months]], _two_.[[Weeks]], _two_.[[Days]], *"day"*, _relativeTo_).
           1. Let _days1_ be _unbalanceResult1_.[[Days]].