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gh-159712: Fix zero-based day rules in zoneinfo - #159723
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TZ string lookups only considered rules from the datetime's year, even when an offset or extended transition time moved a rule into a different year. Consider neighboring DST periods and join touching periods so that permanent DST does not introduce a gap or fold at New Year. Determine folds from actual offset changes. This also avoids marking times as ambiguous when the first occurrence would have fallen in a gap at the start of a short DST period. Keep the Python single-year lookup away from year boundaries when the rule order cannot change between years, to avoid scanning neighboring periods for ordinary footer lookups. Add regressions for both implementations, including positive and negative DST, extended transition hours, and datetime's extreme years.
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Numeric TZ day rules count January 1 as day zero, but both implementations were using the same year baseline as one-based Julian rules. Account for the difference so numeric rules no longer transition one day early. Add local-time and UTC conversion cases for numeric rules, including gaps, folds, leap days and the first and last days of the year.
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October 19, 2026 20:08
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Numeric TZ day rules currently transition one day early because both implementations use the same year baseline as one-based Julian rules. This accounts for the difference and adds local-time and UTC conversion tests for gaps, folds, leap days and the first and last days of the year, including day 365 of a common year falling on January 1 of the next year.
Fixes #159712. This depends on #159722, because correcting day zero exposes the year-boundary bug in permanent DST rules such as EST5EDT,0/0,J365/25. This draft includes that PR's commits until it is merged; the zero-based-day change is isolated in 5b2ce220a8b4a525d8d78d2caf4139baa844aa5d.
The new cases fail without the fix in both implementations. With the year-boundary fix and this change, ./python -m test test_zoneinfo passes all 285 tests, with 30 skips, on a debug build.