types: interpret timestamp in DST transition backwards consistently as the first occurrence

[mjonss]

What problem does this PR solve?

Issue Number: close #61368

Problem Summary: TiDB uses golang time.Date(location) for converting from a datetime + time zone to timestamp, which is not defined how it will interpret datetimes in the range when Daylight Saving Time ends, and the clock moves backwards, i.e. when a time happens twice. This PR defines it as always interpret it as the first time.

Example, we are using 'Europe/Amsterdam' time_zone and insert '2025-10-26 02:30:00', we cannot know if it should be interpreted as '02:30:00+02:00' (first occurrence, still in DST) or '02:30:00+01:00' (second occurrence, now in normal time). This PR will consistently interpret it as '02:30:00+02:00', same as MySQL seems to do.

What changed and how does it work?

Check List

Tests

• [ ] Unit test
• [x] Integration test
• [ ] Manual test (add detailed scripts or steps below)
• [ ] No need to test

  • [ ] I checked and no code files have been changed.

Side effects

• [ ] Performance regression: Consumes more CPU
• [ ] Performance regression: Consumes more Memory
• [ ] Breaking backward compatibility

Documentation

• [ ] Affects user behaviors
• [ ] Contains syntax changes
• [ ] Contains variable changes
• [ ] Contains experimental features
• [ ] Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

When converting to TIMESTAMP, if the value is in the range when transitioning from Daylight Saving Time to normal time (i.e. the clock moves back) always interpret the time as if t was in normal time.

[ti-chi-bot[bot]]

Skipping CI for Draft Pull Request. If you want CI signal for your change, please convert it to an actual PR. You can still manually trigger a test run with /test all

[mjonss]

/retest

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[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 73.5775%. Comparing base (805da7c) to head (c51c3c3). :warning: Report is 94 commits behind head on master.

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Additional details and impacted files

@@               Coverage Diff                @@
##             master     #61447        +/-   ##
================================================
+ Coverage   72.7284%   73.5775%   +0.8491%     
================================================
  Files          1848       1727       -121     
  Lines        498383     479947     -18436     
================================================
- Hits         362466     353133      -9333     
+ Misses       113900     105315      -8585     
+ Partials      22017      21499       -518     

Flag	Coverage Δ
integration	42.4748% <100.0000%> (?)

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Components	Coverage Δ
dumpling	52.7804% <ø> (-0.0897%)	:arrow_down:
parser	∅ <ø> (∅)
br	47.4489% <ø> (+1.0482%)	:arrow_up:

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[mjonss]

/retest

[tiprow[bot]]

@mjonss: Cannot trigger testing until a trusted user reviews the PR and leaves an /ok-to-test message.

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[mjonss]

/retest

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@mjonss: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
idc-jenkins-ci-tidb/unit-test	c51c3c36acf461e600f4af6526571ed99e777db8	link	true	/test unit-test
pull-integration-realcluster-test-next-gen	c51c3c36acf461e600f4af6526571ed99e777db8	link	true	/test pull-integration-realcluster-test-next-gen
pull-unit-test-next-gen	c51c3c36acf461e600f4af6526571ed99e777db8	link	true	/test pull-unit-test-next-gen

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