JST to AEST

How far apart Tokyo and Sydney really are

The difference between these two zones is not a single number. Across 2026 it takes 2 different values — 1 hour, 2 hours — because Sydney observes daylight saving and Tokyo does not.

Working-hours overlap

Treating 09:00–17:00 as the working day in both places, there are 6 hours of overlap: 9:00 am, 10:00 am, 11:00 am, 12:00 pm, 1:00 pm, 2:00 pm in Tokyo, which is 11:00 am, 12:00 pm, 1:00 pm, 2:00 pm, 3:00 pm, 4:00 pm in Sydney. 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 Tokyo and Sydney changes during the year, an offset recorded as a number is only correct until the next transition. Store Asia/Tokyo and Australia/Sydney — 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 Tokyo, Sydney 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.