UTC to JST

How far apart UTC and Tokyo really are

Tokyo is 9 hours ahead of UTC, 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

There is no overlap between a 09:00–17:00 day in UTC and the same window in Tokyo — the zones are 9 hours apart. A recurring call has to sit in one side's early morning or late evening, so the fair arrangement is usually to alternate which side absorbs it.

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 UTC 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 UTC, 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.