IST to JST

How far apart Kolkata and Tokyo really are

Tokyo is 3 hours 30 minutes ahead of Kolkata, and that holds for every day of the year. Neither zone shifts relative to the other, so a single remembered offset is safe for this pair — which is unusual, and worth knowing.

Working-hours overlap

Treating 09:00–17:00 as the working day in both places, there are 5 hours of overlap: 9:00 am, 10:00 am, 11:00 am, 12:00 pm, 1:00 pm in Kolkata, which is 12:30 pm, 1:30 pm, 2:30 pm, 3:30 pm, 4:30 pm in Tokyo. Those are the slots to aim for; everything else costs someone their evening.

Why you should store the zone, not the offset

Even though this pair happens to keep a constant gap today, that is a property of current legislation rather than a guarantee. Store Asia/Kolkata and Asia/Tokyo — 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 Kolkata, Tokyo 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.