KITFORMA · EDITORIAL ANSWER
Why does adding 24 hours not always mean the same local time tomorrow?
KitForma editorial guide. Elapsed duration and calendar-local scheduling are different operations around offset transitions. A fixed offset is not the complete rule set of a named timezone.
Step-by-step answer
Elapsed duration and calendar-local scheduling are different operations around offset transitions. A fixed offset is not the complete rule set of a named timezone.
For elapsed work, add the duration to an instant. For tomorrow’s local appointment, compute the intended local date/time and resolve it using the relevant timezone rules, including invalid or ambiguous times. Retain the zone identifier alongside recurring schedules.
Test dates on both sides of daylight-saving transitions in an applicable zone. DateTimeOffset identifies an instant and offset but does not by itself retain the timezone’s future rules. Never promise that all regions change offsets or use the same transition dates.
Sources and verification
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