Normative: [Alt PR] Fix intermediate value in ZonedDateTime difference #2810
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Codecov ReportAll modified and coverable lines are covered by tests ✅
Additional details and impacted files@@ Coverage Diff @@
## main #2810 +/- ##
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- Coverage 96.49% 96.46% -0.03%
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Files 23 23
Lines 12258 12310 +52
Branches 2265 2271 +6
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+ Hits 11828 11875 +47
- Misses 369 374 +5
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To find the difference between two ZonedDateTimes, we first find the
calendar difference between their date parts. Then we find the time
difference between an intermediate ZonedDateTime value (calculated by
adding the calendar parts to the first ZonedDateTime) and the second
ZonedDateTime.
Previously we would calculate the intermediate value by adding years,
months, and weeks from the date difference, because the days were
calculated as part of the time difference.
This was incorrect, because the intermediate value can shift if it falls
in the middle of a DST transition. However, that could be on a completely
different date than would be expected, leading to surprising results like
this:
const duration = Temporal.Duration.from({ months: 1, days: 15, hours: 12 });
const past = Temporal.ZonedDateTime.from('2024-02-10T02:00[America/New_York]');
const future = past.add(duration);
duration // => 1 month, 15 days, 12 hours
past.until(future, { largestUnit: 'months' }) // => 1 month, 15 days, 11 hours
This result would occur because of a DST change on 2024-03-10T02:00,
unrelated to either the start date of 2024-02-10 or the end date of
2024-03-25. With this change, the intermediate value occurs on
2024-03-25T02:00 and would only shift if that was when the DST change
occurred.
Fixes bug mentioned here: tc39#2760 (comment)
ptomato
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ptomato
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Mar 30, 2024
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Thanks very much for doing this. I've added the spec text.
Tests are consolidated in tc39/test262#4012 now. I added a couple of extra regression tests for the 1 month / 30 days 23 hours bug that this latest update fixed.
On Monday, unless anyone hollers for more time, I'll plan to merge the test262 tests, update the test262 ref in this PR, and then merge this one.
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This PR supersedes #2760
@ptomato
Test262 branch with updated tests:
https://github.com/fullcalendar/test262/tree/temporal-2760
Diff from main: tc39/test262@main...fullcalendar:test262:temporal-2760
Diff from previous: ptomato/test262@temporal-2760...fullcalendar:test262:temporal-2760
I have not updated the spec text. I'd really appreciate some help with that.
Plz lmk if questions. Thanks!