Why some local times do not exist
When clocks move forward, a range of wall-clock times is skipped. In New York on March 8, 2026, the clock moves from 01:59:59 to 03:00:00, so a time such as 02:30 never occurs. This is called a DST gap or a nonexistent local time.
Why some local times happen twice
When clocks move backward, part of the clock is repeated. In New York on November 1, 2026, 01:30 occurs once under daylight time and again under standard time. A date, clock time and zone are therefore insufficient to choose one instant without an explicit offset or disambiguation rule.
Why this matters in software
Scheduling systems should store an instant for completed events and retain an IANA time-zone identifier for future recurring events. Silently choosing the earlier or later occurrence can move payments, jobs, alarms and meetings by an hour. Validate user-entered local times before converting them to UTC.
Frequently asked questions
What is an ambiguous local time?
It is a wall-clock date and time that maps to more than one UTC instant, usually during the autumn clock change.
What is a nonexistent local time?
It is a clock reading skipped when the local offset moves forward. No UTC instant maps back to that local reading.
Do all DST changes move by one hour?
No. The size and timing of clock changes are determined by regional rules and can differ. This checker derives possibilities from the selected IANA zone instead of assuming a one-hour change.
Does the checker use a server?
No. It uses the time-zone rules provided by your browser. Results can therefore depend on how recently the device's time-zone data was updated.
Time-zone calculations use browser-provided IANA rules. Read our methodology, data sources, DST dashboard, or the IANA Time Zone Database.