Polyfill: Extend transition-finding lookahead

GetNamedTimeZonePreviousTransition currently has an optimization to
avoid fruitlessly searching for a transition for far-future dates. This
optimization ignores cases where a time zone's offset changes are
scheduled more than one year in advance.  This commit extends the
lookahead period to three years.

Similarly, GetNamedTimeZoneNextTransition has an optimization
where it assumes that no time zone has any transitions more than
one year in the future. This commit extends this lookahead to three
years later than either the current date or the input instant, whichever
is later.

When testing locally, these changes slow down these AOs by 2%-4%,
which IMO is an OK tradeoff for more accuracy.

Fixes #2564

Author: justingrant

polyfill/lib/ecmascript.mjs

@@ -2859,7 +2859,13 @@ export function GetNamedTimeZoneNextTransition(id, epochNanoseconds) {
   if (epochNanoseconds.lesser(BEFORE_FIRST_DST)) {
     return GetNamedTimeZoneNextTransition(id, BEFORE_FIRST_DST);
   }
-  const uppercap = SystemUTCEpochNanoSeconds().plus(DAY_NANOS.multiply(366));
+  // Optimization: the farthest that we'll look for a next transition is 3 years
+  // after the later of epochNanoseconds or the current time. If there are no
+  // transitions found before then, we'll assume that there will not be any more
+  // transitions after that.
+  const now = SystemUTCEpochNanoSeconds();
+  const base = epochNanoseconds.greater(now) ? epochNanoseconds : now;
+  const uppercap = base.plus(DAY_NANOS.multiply(366 * 3));
   let leftNanos = epochNanoseconds;
   let leftOffsetNs = GetNamedTimeZoneOffsetNanoseconds(id, leftNanos);
   let rightNanos = leftNanos;
@@ -2884,15 +2890,15 @@ export function GetNamedTimeZoneNextTransition(id, epochNanoseconds) {
 }
 
 export function GetNamedTimeZonePreviousTransition(id, epochNanoseconds) {
-  // Optimization: if the instant is more than a year in the future and there
-  // are no transitions between the present day and a year from now, assume
-  // there are none after
+  // Optimization: if the instant is more than 3 years in the future and there
+  // are no transitions between the present day and 3 years from now, assume
+  // there are none after.
   const now = SystemUTCEpochNanoSeconds();
-  const yearLater = now.plus(DAY_NANOS.multiply(366));
-  if (epochNanoseconds.gt(yearLater)) {
-    const prevBeforeNextYear = GetNamedTimeZonePreviousTransition(id, yearLater);
-    if (prevBeforeNextYear === null || prevBeforeNextYear.lt(now)) {
-      return prevBeforeNextYear;
+  const lookahead = now.plus(DAY_NANOS.multiply(366 * 3));
+  if (epochNanoseconds.gt(lookahead)) {
+    const prevBeforeLookahead = GetNamedTimeZonePreviousTransition(id, lookahead);
+    if (prevBeforeLookahead === null || prevBeforeLookahead.lt(now)) {
+      return prevBeforeLookahead;
     }
   }