GMT to IST

How far apart London and Kolkata really are

The difference between these two zones is not a single number. Across 2026 it takes 2 different values — 4 hours 30 minutes, 5 hours 30 minutes — because London observes daylight saving and Kolkata does not.

Working-hours overlap

Treating 09:00–17:00 as the working day in both places, there are 4 hours of overlap: 9:00 am, 10:00 am, 11:00 am, 12:00 pm in London, which is 1:30 pm, 2:30 pm, 3:30 pm, 4:30 pm in Kolkata. 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 London and Kolkata changes during the year, an offset recorded as a number is only correct until the next transition. Store Europe/London and Asia/Kolkata — 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 London, Kolkata 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.