LibTimeZone: Improve the handling of outdated timezone rules #26504
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educasadei
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Dec 23, 2025
AK/Time.h
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| constexpr Tuple<T, u8, u8> seconds_since_epoch_to_date(T seconds) | ||
| { | ||
| constexpr auto seconds_per_day = 60 * 60 * 24; | ||
| T day_offset_to_epoch = seconds >= 0 ? seconds / seconds_per_day : floor_div(seconds, seconds_per_day); |
…o_date() The seconds_since_epoch_to_year() used an innacurate aproximation, causing problems near year boundaries. The new version relies upon the days_since_epoch_to_date() which uses a much better algorithmn.
The previous method simply picked the last rule when all of them were outside the time range, causing the timezone to be permanently in the wrong offset when the last rule didn't have an offset of 0. One of the affected timezones was America/Sao_Paulo, because Brazil's DST rules end in 2019 and its last rule is setting the start of DST for 2018, which (due to the behavior described above) causes the timezone to be stuck in Daylight Saving Time. This surfaced an error in two of the test cases for Europe/Moscow, they were using the incorrect daylight and standard time names for the period being tested.
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The previous method simply picked the last rule when all of them were outside the time range, causing the timezone to be permanently in the wrong offset when the last rule didn't have an offset of 0.
One of the affected timezones was America/Sao_Paulo, because Brazil's DST rules end in 2019 and its last rule is setting the start of DST for 2018, which (due to the behavior described above) causes the timezone to be stuck in Daylight Saving Time.
This surfaced an error in two of the test cases for Europe/Moscow, they were using the incorrect daylight and standard time names for the period being tested.