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UTC vs GMT: what is the difference?

By the TimeSlotSync team · Last reviewed 12 October 2026 · Time zone data: IANA tz database 2026c

A time standard, a time zone, and the summer-time mistake that catches people out every year.

UTC and GMT are often used as if they meant the same thing, and for scheduling a meeting the difference is almost never more than a second. The confusion that matters is different: the UK is not on GMT for half the year, so "3 pm GMT" in an invite sent in July is very likely wrong.

The short answer

• UTC (Coordinated Universal Time) is the world's reference time standard. It is not a time zone that any country "lives in"; it is the zero point that every time zone is defined against. It never changes for daylight saving.
• GMT (Greenwich Mean Time) is a time zone: the one used by the United Kingdom, Ireland, Portugal, Iceland and much of West Africa in winter (Iceland and many West African countries use it all year). In practice it shows the same clock reading as UTC.
• BST (British Summer Time) is what the UK uses from the last Sunday of March to the last Sunday of October. It is UTC+1. Ireland calls its summer time IST, Irish Standard Time, also UTC+1.

Where the two came from

GMT is the older idea. It was the mean solar time at the Royal Observatory in Greenwich, and in 1884 the International Meridian Conference chose Greenwich as the prime meridian for the world. For most of the 20th century, GMT was the reference that other time zones were measured from.

Solar time has a problem, though. The Earth's rotation is slightly irregular and gradually slowing down, so a day measured by the sun is not perfectly constant. Atomic clocks, which arrived in the 1950s, are far more stable. UTC was introduced in the 1960s to combine the two. It ticks at the steady rate of International Atomic Time, but it is kept within 0.9 seconds of solar time (UT1) by occasionally inserting a leap second.

In 2022 the General Conference on Weights and Measures voted to stop inserting leap seconds by 2035, letting UTC drift further from solar time. For anyone booking meetings, none of this is visible: UTC and GMT will read the same on your clock for the foreseeable future.

Why it matters in an invite

The practical mistake happens between late March and late October. During those months:

• London is on BST, UTC+1, not GMT.
• An invite that says "15:00 GMT" means 16:00 in London, which is probably not what the sender meant.
• Someone in New York converting "15:00 GMT" correctly gets 11:00 EDT, while a colleague who assumes the sender meant London time gets 10:00 EDT. Two people now expect two different start times.

If you want to say "London time", write "London time", or "UK time". If you want a fixed reference that never changes, write UTC, and convert it for your attendees.

UTC in software, logs and aviation

• ISO 8601, the international date format, marks UTC with a trailing Z: 2027-03-28T01:00:00Z is the exact moment Europe switches to summer time in 2027.
• "Zulu time" is the same thing in aviation and military use: "Z" is "Zulu" in the NATO phonetic alphabet.
• Servers and databases should store timestamps in UTC and convert to local time only for display. Storing local time loses information: 01:30 on 1 November 2026 happens twice in New York.
• Calendar files (.ics) either store times in UTC or attach an explicit time zone. TimeSlotSync's calendar export writes UTC, so every calendar app converts it the same way.

Quick reference

Related guides

• Time zone abbreviations that cause mix-ups
• Daylight saving time 2026–2027: every clock change