Daylight Saving and Time Zone Conversion
Why the gap between two cities changes through the year, and how converters handle DST gaps and repeats.
Offsets are not fixed
A time zone is not a single offset from UTC. Most zones change offset at least once a year for daylight saving, and some have changed their rules entirely over the decades. That is why the difference between, say, London and New York is five hours in winter and four hours in summer — the cities do not move, their offsets do.
Gaps and repeated hours
When clocks spring forward, an hour of local wall time simply does not exist — 2:30 AM may never occur on that date. When clocks fall back, an hour occurs twice. A careful converter does not pretend otherwise: it relies on the platform's IANA time zone data, which records every change, and it states the zone rather than guessing an offset. If you schedule across a DST change, confirm the meeting in both zones and in UTC.
A safe habit
For anything that must happen at an exact instant — a launch, a deadline, a live event — agree the time in UTC as well as in local time. UTC never observes daylight saving, so it removes the ambiguity entirely.
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