UTC, GMT, Zulu, and BST turn up in the same inbox all the time: a monitoring alert stamped 14:00 UTC, a flight plan reading 1400Z, a shipping notice in GMT, and a London colleague saying “let’s meet at 3.” Three of those four labels mean the same instant. The fourth is the one that breaks calendars.
This guide sorts out which labels are interchangeable, which one is not, and why the distinction survives even though the arithmetic mostly does not.
The short answer
| Label | Offset | Follows daylight saving? | Where you see it |
|---|---|---|---|
| UTC | UTC+0 | No | APIs, logs, databases, ISO-8601 timestamps |
| GMT | UTC+0 | No, as a time standard | Maritime, broadcast, older documents, some UK copy |
| Zulu (Z) | UTC+0 | No | Aviation, military, METAR/TAF weather reports |
| BST | UTC+1 | Yes — it is the UK summer setting | UK wall clocks, late March to late October |
For conversion purposes, UTC, GMT, and Zulu are one thing. BST is a different offset and the source of most real errors.
UTC: the modern standard
Coordinated Universal Time is the reference the world’s clocks are actually synchronised against. It is maintained from atomic clocks, it never observes daylight saving, and it is what an ISO-8601 timestamp such as 2026-07-20T14:00:00Z refers to.
The odd acronym comes from a compromise: English speakers proposed CUT, French speakers TUC, and the standards bodies settled on UTC so no language got its preferred initialism. If a system, contract, or log format gives you a choice, UTC is the label to write.
GMT: the historical standard that became a time zone name
Greenwich Mean Time predates UTC and was defined astronomically, from the mean solar day at the Greenwich meridian. UTC superseded it as the technical reference in 1972, but GMT never went away, and this is where the confusion begins: GMT now does double duty.
- As a time standard, GMT is UTC+0 and never moves.
- As a UK time zone name, GMT is what British clocks read in winter, switching to BST in summer.
So “GMT” on a maritime notice or a broadcast schedule means UTC+0 year-round, but “we’re on GMT” from a London colleague in July is simply wrong — they are on BST. This site treats GMT strictly as the fixed UTC+0 standard, which is why a London summer wall clock will not match it.
Zulu: the same instant, spoken aloud
Aviation and military communication assigns a letter to each hourly offset, and UTC+0 drew Z. Because radio operators need a word that cannot be misheard, Z is spoken “Zulu” from the NATO alphabet. 1400Z, “fourteen hundred Zulu,” and 14:00 UTC are identical.
You will see Zulu time on flight plans, NOTAMs, METAR and TAF weather reports, and in ISO timestamps, where the trailing Z is the same convention. If a pilot briefing and a server log disagree, it is not the label — check the date rollover instead.
BST: the one that is genuinely different
British Summer Time is UTC+1, running from the last Sunday in March to the last Sunday in October. It is the single most common source of GMT-related mistakes, because the UK is the country people associate with GMT and therefore assume is always on it.
A worked example of the error:
- A London colleague proposes 12:00 noon their time in August.
- Treated as GMT (UTC+0), that converts to 8:00 AM Eastern Daylight Time.
- It is actually BST (UTC+1), so noon London is 11:00 UTC, which is 7:00 AM Eastern.
One hour off, in the direction that makes you late. The same trap applies to Ireland (IST, also UTC+1 in summer) and Portugal (WEST). The fix is not arithmetic but habit: ask for a city or an IANA zone name such as Europe/London, never a bare three-letter label, and let the zone database decide whether that date falls in GMT or BST.
Which label should you write?
- Writing for systems — use UTC, and prefer a full ISO-8601 stamp with the
Zsuffix. Store UTC, render local. - Writing for aviation or maritime readers — use Zulu or GMT to match the convention they already read, and note UTC equivalence once.
- Writing for UK readers — never write GMT for a summer date. Write “London time” or the explicit label for the season.
- Writing for a mixed audience — give UTC plus one or two named-city translations, and link to a converter for everyone else.
The leap second footnote
UTC and astronomical time drift apart because the Earth’s rotation is not perfectly constant. Leap seconds were introduced to keep UTC within 0.9 seconds of solar time; 27 have been added since 1972, all as an extra second at the end of a June or December day. In 2022 the international bodies responsible voted to stop the practice by 2035, largely because leap seconds cause real outages in systems that assume every minute has sixty seconds.
None of this affects meeting scheduling. It matters only if you write software that must not break on a 61-second minute, in which case your platform’s smearing behaviour is worth reading up on.
Convert a UTC, GMT, or Zulu timestamp to U.S. clocks on the UTC to EST and UTC to PST pages.
Practical summary: treat UTC, GMT, and Zulu as three names for one instant, and treat BST as a separate offset that a UK date silently switches into for seven months of the year. Naming a city instead of an abbreviation removes the whole class of error.