Time zones and daylight saving: why some times do not exist
How UTC offsets change through the year, why some local times are skipped or repeated, and how to convert meeting times safely.
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Offsets are not fixed
A time zone such as America/New_York is not the same as a fixed offset like UTC−5. For part of the year New York observes daylight saving time and is UTC−4. The dates of the switch differ between countries, and the rules themselves change from time to time by law.
That is why reliable software uses the IANA time zone database — names like Europe/London or Asia/Kolkata — rather than abbreviations like EST or BST, which are ambiguous and do not record when daylight saving starts or ends.
Skipped and repeated hours
When clocks spring forward, an hour of local time is skipped. In the UK in 2026, clocks jump from 01:00 to 02:00 on 29 March, so 01:30 that morning never happens. When clocks fall back, an hour repeats: on 25 October 2026, 01:30 occurs twice in the UK, once in summer time and once in winter time.
A careful converter has to report these cases rather than silently guessing. Numerith’s time zone calculator flags nonexistent times and, for repeated times, shows both possible instants.
Scheduling across zones
When planning a recurring meeting, check the times in all participants’ zones for dates on both sides of each daylight-saving change — the gap between two cities can change by an hour for a few weeks a year when they switch on different dates. The UTC offset comparison tool shows every zone at one instant, with its daylight-saving status.