UTC to AEST

How far apart UTC and Sydney really are

The difference between these two zones is not a single number. Across 2026 it takes 2 different values — 10 hours, 11 hours — because Sydney observes daylight saving and UTC does not.

Working-hours overlap

There is no overlap between a 09:00–17:00 day in UTC and the same window in Sydney — the zones are 11 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

Because the gap between UTC and Sydney changes during the year, an offset recorded as a number is only correct until the next transition. Store UTC 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 UTC, 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.