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time.timezone/time.altzone are computed once at process start from TimeZoneInfo.Local.BaseUtcOffset, which models a zone as having at most two offsets (standard/daylight) for all time. That breaks down when a jurisdiction permanently abolishes DST mid-year (e.g. America/Vancouver switching to a DST-less "PT" zone on 2026-11-01): mktime() computed UTC using the stale, zone-wide offset instead of the offset actually in effect on the given date, and produced timestamps off by up to an hour around the transition.
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time.timezone/time.altzoneare computed once at process start fromTimeZoneInfo.Local.BaseUtcOffset, which models a zone as having at most two offsets (standard/daylight) for all time. That breaks down when a jurisdiction permanently abolishes DST mid-year (e.g. America/Vancouver switching to a DST-less "PT" zone on 2026-11-01):mktime()computed UTC using the stale, zone-wide offset instead of the offset actually in effect on the given date, and produced timestamps off by up to an hour around the transition.I expect more jurisdictions to follow (e.g. Alberta abolishes the DST switch too).