What is Zulu time?

The military zone system gives each hour of offset a letter. Z is the zero offset, A through M run east of it and N through Y run west, with J skipped so it can mean local time. Z is read out as Zulu to keep it clear over a radio. So a NATO message, a flight plan and a weather report all write a UTC time as digits followed by Z: 0315Z is 03:15 UTC.

The civilian standard picked up the same letter. ISO 8601 allows Z in place of an offset, which is why 2026-06-01T11:10:52Z is the form most APIs and log files emit. Nobody on a flight deck needs to know the local offset to act on it.

Aviation runs on it. Flight plans, ATC clearances, NOTAMs and METAR observations are all in UTC, so a flight crossing six time zones has one clock. A METAR header such as KJFK 011152Z means an observation at JFK on the 1st of the month at 11:52 UTC. The military uses it for the same reason, written as a date-time group like 011152Z JUN 26. Weather services issue warnings in Zulu. GPS operations and amateur radio logs use it.

In software, Z shows up at the end of ISO 8601 strings from JavaScript's toISOString(), from most cloud APIs and in database exports, which is where a developer usually meets it. The current time in UTC is the current Zulu time.

UTC and GMT agree to within a second, and for every practical purpose a time in one is the same in the other. They are not the same kind of thing. GMT is a timezone, the local time of Greenwich, which the UK uses in winter and swaps for BST in summer. UTC is a standard kept by atomic clocks, with leap seconds added so that it stays within 0.9 seconds of the Earth's rotation. Zulu means UTC, not GMT, so a Zulu time never shifts for British summer time.

If you want to be exact, write UTC in documentation and Z in data. If someone says GMT and means the offset rather than London's clock, the numbers still work out.

Add or subtract your offset from UTC. 1152Z is 07:52 in New York during summer, when the offset is UTC−4, and 06:52 in winter at UTC−5. In Tokyo it is 20:52 all year. Two things trip people up: the sign, since zones west of Greenwich subtract, and daylight saving, which changes the amount twice a year across most of Europe and North America. A converter removes both.

Paste 2026-06-01T11:10:52Z below and parsetime shows it in your timezone, in UTC and in other cities. The UTC to EST page covers the US Eastern case with an hour table, and the ISO 8601 converter turns a local time back into a Z string.