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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.
pganssle
commented
Oct 19, 2026
pganssle
commented
Oct 19, 2026
pganssle
commented
Oct 19, 2026
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Fixes #159715.
TZ footer rules can put a transition in a different year from the rule that defines it. The lookup now considers neighboring DST periods, including periods that touch to represent permanent DST. This fixes incorrect offsets and UTC round trips near New Year in both implementations. Folds are based on an actual decrease in offset, so dummy transitions between permanent-DST periods don't create a fold.
The tests cover positive and negative DST, displaced start and end rules, extended transition hours, years 1 and 9999, and short DST periods where only part of the nominal fold is ambiguous. The Python implementation retains a single-year lookup away from year boundaries when the rules are far enough apart that their order cannot change between years.
Validation:
./python -m test test_zoneinfopasses on a debug build. The year-boundary regressions fail with the original implementations restored.ZoneInfomishandles TZ footer rules spanning a year boundary #159715