DST Inspector

Key point

skipped and repeated local times

Twice a year most timezones skip an hour or repeat one. Inside the skipped hour a local time does not exist; inside the repeated one it happens twice.

Pick a zone and a year to see its transitions, then drag the ticker into a gap (the skipped hour) or an overlap (the repeated hour) and watch convertPlainDateTimeToZoned resolve the same local wall-clock time four different ways, one per disambiguation option.

The disambiguation option belongs to functions that set a local time, because a local time can be skipped or repeated. Functions that find a boundary, like startOfZoned, do not take it: they always return the earliest instant of the unit.

1. Find transitions

Year

Type	Local Date	Local Hour	UTC Instant	Offset Before → After

2. Probe a moment

A local hour that was skipped during spring-forward — requires disambiguation.

Disambiguation: compatible takes the later instant in a gap and the earlier one in an overlap. earlier and later take that instant. reject gives no result.

 

See it in action: 1:30am on 3 November 2024 in New York happens twice — scrub across the transition and switch disambiguation between earlier, later and reject to see which instant each one picks.

Reference: getDstTransitions finds the transitions; convertPlainDateTimeToZoned resolves the probe.