Interactive publication
Back to the catalogueDSTCase 02 of 06
Fall-back fold (ambiguous local time)
When clocks fall back, a local wall time occurs twice (e.g. 01:30 on 2026-11-01 in America/New_York), once at the pre-transition offset and once at the post-transition offset — two different UTC instants, two different slots.
Class
Case to account for
Affects
3 conversions
01
Detection
Two instants one hour apart share the same wall time with different offsets.
02
Mitigation
01
Declare disambiguation: earliest (first occurrence), latest (second), or reject.
02
Record wasAmbiguous and which policy was applied.
03
Affected conversions
Timestamp + zonehour-of-week slot
OPENBroadcast day (declared cutover) ⇄ slot-set
OPENDaypart (local band) ⇄ UTC slot-set
OPEN