[JSC] Date: convert a local time to UTC with the zone's own offset when the local time is outside the time value range - #615
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I reviewed this PR and didn't find any bugs. Because it changes spec-defined UTC(t) behavior in JSDateMath.cpp and widens the clamp in WTF's date parser — both CODEOWNERS-covered runtime paths whose edge cases are subtle — a human look would still be worthwhile.
What was reviewed:
DSTCache::localTimeOffsetnow bypasses the cache and calls ICU directly for out-of-range inputs instead ofequivalentTime(); the in-range cache path is untouched.localTimeToMSrefactor: the!(abs(ms) <= max + msPerDay)guard correctly catches NaN/±inf as before and additionally prevents the previously-UBint64_tcast for huge finite inputs;gregorianDateTimeToMSand bothparseDatepaths preserve the non-LocalTime passthrough.ymdhmsToMillisecondswidening plustimeClipin the WTFparseDate(span)overload keeps the final result clamped for external callers.- New stress test follows JSTests conventions (throws on failure, no logging, uses
$vm.setHostTimeZone+setTimeoutto pick up zone changes on VM entry).
Extended reasoning...
Overview
The PR fixes local-to-UTC conversion at the ±8.64e15 ms time-value boundaries. In Source/JavaScriptCore/runtime/JSDateMath.cpp, DSTCache::localTimeOffset stops mapping out-of-range inputs through WTF::equivalentTime() (which applied a 2008-2035 year's offset) and instead asks ICU directly, since the DST cache's empty-entry sentinels are the range endpoints and cannot represent entries outside them. localTimeToMS becomes the single UTC(t) implementation with an early PNaN when |ms| > maxECMAScriptTime + msPerDay, guarding the double→int64_t cast; gregorianDateTimeToMS and both parseDate code paths now delegate to it. In Source/WTF/wtf/DateMath.cpp, ymdhmsToMilliseconds widens its clamp by one day so a parsed local time just outside the range survives until the offset is subtracted, and the WebCore-facing parseDate(span) overload adds an isfinite guard and applies timeClip to its result. A new stress test round-trips near-limit time values through the constructor, every local setter, and Date.parse across 13 zones.
Security risks
None identified. This is numeric edge-case handling in date math with no auth, crypto, permission, or untrusted-input parsing beyond what already existed. The change actually removes one instance of undefined behavior: the old code did static_cast<int64_t>(localTimeResult) on arbitrary finite doubles (e.g. new Date(1970, 0, 1, 0, 0, 0, 1e300)), which the new range guard now short-circuits to PNaN before the cast. The ICU call in the new bypass path (calculateLocalTimeOffset) is the same one the cache already invoked on misses.
Level of scrutiny
Moderate-to-high. The diff is small and the reasoning in the comments is sound, but it alters observable Date semantics at the spec's boundary conditions across every local setter, the multi-arg constructor, and Date.parse, and it touches the DST offset cache that all local Date operations flow through. /Source/JavaScriptCore and /JSTests are listed under @ WebKit/jsc-reviewers in .github/CODEOWNERS, and per the review guidelines CODEOWNERS-covered runtime changes should get a human sign-off rather than an auto-approve.
Other factors
I traced the refactor for behavior preservation: for TimeType::UTC inputs localTimeToMS returns the value unchanged, matching the old gregorianDateTimeToMS fall-through; for NaN/±inf the !(abs <= …) predicate is true so PNaN is returned, which every caller then timeClips to NaN as before. The V8 parser path previously cast value to int64_t without a finiteness check — routing it through localTimeToMS fixes that too. The WTF parseDate(span) overload now timeClips its result, which is a semantic tightening for non-JSC callers; that seems intentional and correct given ymdhmsToMilliseconds no longer clamps to the exact range. The new stress test throws on failure, prints nothing on success, and skips on PlayStation (no $vm.setHostTimeZone there), consistent with JSTests/CLAUDE.md. The PR reports full test262/mozilla/stress parity, but I did not independently run those, which is another reason a human confirmation is appropriate.
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No actionable comments were generated in the recent review. 🎉 ℹ️ Recent review info⚙️ Run configurationConfiguration used: Organization UI Review profile: ASSERTIVE Plan: Essentials Run ID: 📒 Files selected for processing (1)
Included review availability: Your plan provides up to 5 included reviews per hour; 1 remains after this review. WalkthroughDate conversion now handles local-time adjustments near ECMAScript time-value limits. Parser paths use shared conversion and clipping logic. A time-zone-aware stress test covers boundary, offset, and out-of-range behavior. ChangesDate limit conversion
Priority: ➖ Normal Merge Risk: ⚪ Minimal · up to This change refines how JavaScriptCore computes local time offsets and parses dates near the edges of the supported date range, using ICU directly for out-of-range values and sharing conversion logic with the parser. The change is small, targeted, and covered by a new boundary-focused stress test, so it should be safe to merge without additional follow-up. 🚥 Pre-merge checks | ✅ 3 | ❌ 1❌ Failed checks (1 warning)
✅ Passed checks (3 passed)
Full details: Description checkExplanation The description gives a detailed problem statement, fix summary, affected behavior, and test results. It does not include the required Bugzilla link, reviewer line, or template-style changed-file and function list.
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Actionable comments posted: 1
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Inline comments:
In `@JSTests/stress/date-local-time-utc-conversion-at-time-value-limits.js`:
- Around line 127-140: Update the asynchronous chain around runNextStep to
capture totalSteps after all steps.push calls, track completed callbacks, and
assert at the end that the completed count matches totalSteps. Preserve the
existing timezone setup and assertions while ensuring the test cannot pass when
no timer callback runs.
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JSTests/stress/date-local-time-utc-conversion-at-time-value-limits.jsSource/JavaScriptCore/runtime/JSDateMath.cppSource/WTF/wtf/DateMath.cpp
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…en the local time is outside the time value range The local Date setters, the multi-argument Date constructor and Date.parse of a string without a UTC offset compute TimeClip(UTC(local time value)). The note on UTC(t), https://tc39.es/ecma262/#sec-utc-t, says the local time value must not be limited to the time value range, because for a Date within one UTC offset of either end of the range it is outside of it. 320788@main (bug 323443) lets such a value through to localTimeOffset() when it is within a day of the range (canNarrowToInt64Milliseconds). DateCache::DSTCache::localTimeOffset() then mapped it through WTF::equivalentTime(), that is to the same month and day in a year between 2008 and 2035, before it asked ICU for the offset, because the offset cache marks an empty entry with the two limits and cannot hold anything outside them. The result used that other year's UTC offset. In America/New_York, which is at its local mean time of -4:56:02 before 1883, d.setHours(d.getHours()) on d = new Date(-8.64e15 + 36e5) moved d by 56 minutes 2 seconds and new Date(-8.64e15).setMilliseconds(0) was NaN. In Pacific/Apia (+13 today, -11 in 2008) every local setter on the last 13 hours of the range gave NaN, and the stress test that 320788@main added, date-constructor-out-of-range-components.js, fails there. V8 asks ICU directly and gets these right. Now localTimeOffset() asks ICU directly, without the cache, for a time outside the range. The useV8DateParser() branch of parseDate() goes through localTimeToMS() like the other local-to-UTC conversions, so it gets the same canNarrowToInt64Milliseconds() guard. WTF's own date parser clipped the local (or pre-offset) value to the time value range in ymdhmsToMilliseconds(), before the string's UTC offset or the local time offset was subtracted, so Date.parse("-271821-04-19T20:00:00-04:00") and, in New York, Date.parse("-271821-04-19T19:03:58") were NaN instead of -8.64e15. That bound is now a day wider, and the WebCore-facing parseDate() overload TimeClips its final value instead. * JSTests/stress/date-local-time-utc-conversion-at-time-value-limits.js: Added. * Source/JavaScriptCore/runtime/JSDateMath.cpp: (JSC::DateCache::DSTCache::localTimeOffset): (JSC::DateCache::parseDate): * Source/WTF/wtf/DateMath.cpp: (WTF::ymdhmsToMilliseconds): (WTF::parseDate):
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…ar the time value limits use the zone's own offset (WebKit bump for oven-sh/WebKit#615) Pin WebKit to the preview build of oven-sh/WebKit#615 and add a test. JSC remapped a local time value outside the time value range to a year between 2008 and 2035 before it asked ICU for the UTC offset, so a local setter or the multi-argument constructor on a valid Date within one UTC offset of either end of the range was off by the difference between the two offsets, or NaN.
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Problem
Datesetters, the multi-argumentDateconstructor andDate.parseof an offset-less string computeTimeClip(UTC(local time value)). For a valid Date within one UTC offset of either end of the time value range (±8.64e15 ms) that local time value is outside the range, and JSC converted it with the wrong UTC offset.TZ=America/New_York:d = new Date(-8.64e15 + 36e5); d.setHours(d.getHours())movesdby 56 min 2 s, andnew Date(-8.64e15).setMilliseconds(0)isNaN.TZ=Pacific/Apia: every local setter on the last 13 hours of the range isNaN, andJSTests/stress/date-constructor-out-of-range-components.js(320788@main) fails. V8 (Node 26) returns the identity in all of these, as the note on UTC(t) requires.DateCache::DSTCache::localTimeOffset()(runtime/JSDateMath.cpp). 320788@main (bug 323443) lets a local time within a day of the range reach it, and it then mapped anything outside[minECMAScriptTime, maxECMAScriptTime]throughWTF::equivalentTime()(same month and day in a year between 2008 and 2035) before it asked ICU, because its offset cache marks an empty entry with those two limits. So the conversion used that other year's offset: EDT -4:00 instead of New York's local mean time -4:56:02, or Apia's 2008 offset of -11:00 instead of +13:00.ymdhmsToMilliseconds()(wtf/DateMath.cpp) clipped the value to the range before the string's UTC offset or the local offset was subtracted, soDate.parse("-271821-04-19T20:00:00-04:00")wasNaNwith--useV8DateParser=0.Fix
DSTCache::localTimeOffset()asks ICU directly, without the cache, for a time outside the range. The cache logic is unchanged for everything inside it.useV8DateParser()branch ofDateCache::parseDate()converts throughlocalTimeToMS(), so it gets the samecanNarrowToInt64Milliseconds()guard as the other local-to-UTC paths from 320788@main.ymdhmsToMilliseconds()accepts values up to a day outside the range, and the WebCore-facingWTF::parseDate(span)overload TimeClips its final value instead.JSTests/stress/date-local-time-utc-conversion-at-time-value-limits.js(new) round-trips the ten time values nearest the limits through the constructor and every local setter in 13 zones, plus pinned New York, Tokyo, Apia and Kiritimati cases and theDate.parseforms. A Release JSCOnly build of this branch passes it with the default options,--useV8DateParser=1,--useJIT=0and eager JIT thresholds, and passesdate-constructor-out-of-range-components.jsunderTZ=Pacific/Apia. The jsc ofmain(4b9ff99) fails the new test in all 13 zones with the default parser (8 through the constructor or a setter, 5 only throughDate.parse), in 8 zones with--useV8DateParser=1, and fails the upstream test under Apia. No change in test262built-ins/Date,annexB/built-ins/Date,intl402/Dateandintl402/DateTimeFormat(1748 runs, all pass under Los_Angeles, New_York, Tokyo and UTC, same results asmainunder Apia), the 150 mozilla Date tests, theJSTests/complextime zone tests, or the 93date-*/intl-date*/temporal-now*stress tests.Background
Dateoperations decomposet + offset(t)into fields, edit a field, recompose a local time valuel, and storeTimeClip(l - offset(l)). Fortnear a limit,lis past it by up to the offset, which is what the spec note is about.UCalendar(ucal_get(UCAL_ZONE_OFFSET / UCAL_DST_OFFSET)for a UTC input,ucal_getTimeZoneOffsetFromLocalfor a local one), behind a per-direction cache of up to 32[start, end] -> offsetintervals ported from V8. ICU answers for any instant in about ±5.8 million years, and for an instant before a zone's first rule it returns the zone's local mean time, which is why very old dates have offsets like -4:56:02.WTF::equivalentTime()exists for the non-ICUlocaltime_r()path inWTF::calculateLocalTimeOffset(), wheretime_tcannot represent such dates. The ICU path never needed it.Notes
autobuild-preview-pr-615-c1a60940) was based on dfd6964 and carried its own ±1-day guard inlocalTimeToMS()/gregorianDateTimeToMS()/parseDate(). Upgrade to upstream WebKit ccdcb8a026 #614 brought in 320788@main, which adds the same guard ascanNarrowToInt64Milliseconds(), so this revision is rebased onmainand keeps only the parts upstream does not have. The observable behavior of the two revisions is the same.TZ=Asia/Manilathe new build differs frommainon test262built-ins/Date/prototype/toISOString/15.9.5.43-0-8.js: that test assumes the sign of the zone's offset at the epoch (+8) also holds at -271821, but Manila's local mean time until 1844 is -15:56:08, sonew Date(1970, 0, -99999999, 0, 0, 0, -1)is a valid Date there. The new result, -8639999942632001, is what V8 returns too. The test passes in every zone whose offset sign did not flip.LocalTime(t)only ever sees a time value inside the range. Only the local-to-UTC direction reached theequivalentTime()branch.mainstill has theequivalentTime()remap and theymdhmsToMilliseconds()clip, so this is a candidate follow-up to bug 323443.