EST to GMT

How far apart New York and London really are

The difference between these two zones is not a single number. Across 2026 it takes 2 different values — 4 hours, 5 hours — because both New York and London observe daylight saving, but they do not switch on the same dates.

Note the short periods above (8 Mar–28 Mar, 25 Oct–31 Oct). That is the window where one zone has already changed its clocks and the other has not. It is the single most common source of missed meetings between these two places, because the offset people carry in their heads is briefly wrong by an hour.

Working-hours overlap

Treating 09:00–17:00 as the working day in both places, there are 3 hours of overlap: 9:00 am, 10:00 am, 11:00 am in New York, which is 2:00 pm, 3:00 pm, 4:00 pm in London. Those are the slots to aim for; everything else costs someone their evening.

Why you should store the zone, not the offset

Because the gap between New York and London changes during the year, an offset recorded as a number is only correct until the next transition. Store America/New_York and Europe/London — the IANA identifiers — alongside any time a user entered locally. The identifier carries the zone's entire history of rule changes, so a date from 1995 converts with the rules that were in force in 1995.

Internally, keep the instant as a Unix timestamp. The epoch value is identical in New York, London and everywhere else; the zone is applied only at the edges, when a human needs to read it. The current Unix timestamp is at the top of this page, and the zones versus offsets guide explains why the distinction bites.