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Back to the catalogueDSTCase 04 of 06
Partial-hour DST shift
Not every DST change is one hour. Lord Howe Island shifts by 30 minutes (+10:30 ↔ +11:00); some historical transitions were 20 or 40 minutes. The gap or fold is then a partial hour, so the ambiguity window is not a whole slot.
Class
Case to account for
Affects
2 conversions
01
Detection
offsetDelta across the transition !== 60 minutes.
02
Mitigation
01
Compute gap/fold magnitude from the tz database; never assume a one-hour shift.
02
A 30-minute fold means only half the hour is ambiguous — apportion accordingly.
03
Affected conversions
Timestamp + zonehour-of-week slot
OPENBroadcast day (declared cutover) ⇄ slot-set
OPEN