The letter that marks UTC in an ISO 8601 timestamp such as 12:00Z is Z.
In ISO 8601 notation, Z is the zone designator for a zero offset from Coordinated Universal Time. It is written directly after the time, without a space, so 09:30 UTC can be represented as 09:30Z. A full timestamp might look like 2026-09-15T12:00:00Z.
The letter is commonly associated with “Zulu,” the phonetic-alphabet word for Z. Zulu time is widely used in aviation, military, maritime, and other international communications because it avoids confusion between local clock times.
Z is not the same as T. In a combined ISO 8601 date-time, T separates the date from the time, as in 2026-09-15T12:00Z. Other time zones use numeric offsets, such as +05:30 or −04:00, rather than Z. The symbol therefore identifies UTC specifically, not every time zone abbreviation.