Historical time zone changes and why they still matter
A timestamp from years ago needs the time zone rule that was actually in effect back then, not today's rule. Getting this wrong quietly corrupts historical data.
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#dst
Countries that have changed their time zone rules
A country's UTC offset is not permanent. Governments periodically redraw zone boundaries, adjust offsets, or add and drop daylight saving, and software has to keep up.
Daylight saving transitions and the requests that break around them
Twice a year, a range of local times either does not exist or exists twice. Software that assumes every clock time is unambiguous will fail exactly then.
Half-hour and 45-minute time zones, explained
Not every time zone sits on a clean hour boundary. Several countries use 30 or 45 minute offsets, and code that assumes whole hours will break on them.
Time zone data: how the rules actually get maintained
Time zone offsets are not fixed physical facts, they are legal decisions that governments change. Here is how that gets tracked and kept current.