UTC vs GMT: What's the Difference?

For reading a clock, UTC and GMT show the same time: 12:00 UTC is 12:00 GMT. The difference is in what they are. UTC (Coordinated Universal Time) is the atomic-clock time standard the world's clocks and computers are set by. GMT (Greenwich Mean Time) began as astronomical time at Greenwich, England, and today is mostly used as the name of the UK's winter time zone. The UK isn't on GMT all year, though. In summer it switches to British Summer Time, an hour ahead. That last point causes far more scheduling mistakes than the technical difference between UTC and GMT.

UTC vs GMT vs UK time at a glance

What GMT is

Greenwich Mean Time was defined by the sun's average ("mean") position as seen from the Royal Observatory in Greenwich, London. Because it follows the Earth's rotation, it inherits the planet's irregularities: the Earth doesn't spin at a perfectly constant rate, so a day measured by the sun isn't always the same length. Scientists now measure this rotation-based time as UT1, which is the modern astronomical successor to GMT.

In everyday use, "GMT" now usually means a time zone: GOV.UK describes it as the time the UK uses once the clocks have gone back in autumn. When someone in London says "3 p.m. GMT" in January, they mean 15:00 UTC.

What UTC is

Coordinated Universal Time is built from atomic clocks. According to NIST, UTC is based on International Atomic Time (TAI) but adjusted by leap seconds so that it stays within ±0.9 seconds of UT1, the time scale that follows the Earth's rotation. In other words, UTC ticks with atomic precision but is nudged now and then so it never drifts far from the sun.

UTC never changes for daylight saving time. It's the fixed reference every time zone is defined against: EST is UTC−5, Tokyo is UTC+9, and so on. That's why aviation, weather services, servers, logs and APIs use UTC.

Leap seconds: the real difference

A leap second is an extra second added to UTC, written as 23:59:60, to keep atomic time in step with the Earth's rotation. Here's what NIST's leap second table shows:

• 27 leap seconds have been added since the first on June 30, 1972.
• The most recent was added on December 31, 2016.
• UTC is currently 37 seconds behind TAI, the uncorrected atomic time scale.

Because of these adjustments, UTC and the astronomical successor to GMT (UT1) are never more than 0.9 seconds apart. For any clock you'll actually look at, that's zero difference.

Leap seconds may be phased out. In 2022, the General Conference on Weights and Measures adopted Resolution 4, which says the maximum allowed difference between UT1 and UTC will be increased in, or before, 2035. That would let UTC run without leap seconds for much longer. The new limit hadn't been set when the resolution was adopted.

UK time isn't always GMT

This is the most practical point in this guide. According to GOV.UK, UK clocks go forward one hour at 1:00 a.m. on the last Sunday in March and back one hour at 2:00 a.m. on the last Sunday in October. Between those dates, the UK is on British Summer Time (BST), which is UTC+1.

So a meeting invite that says "10:00 GMT" in July is ambiguous. Does the sender mean 10:00 UTC, or 10:00 London time (09:00 UTC)? If you mean UTC, write UTC. If you mean London time, write "UK time" or "London time."

The weeks when New York and London are 4 hours apart

The US and the UK change their clocks on different dates. The US switches on the second Sunday in March and the first Sunday in November, while the UK uses the last Sundays in March and October. During the weeks in between, the usual 5-hour gap between New York and London drops to 4 hours:

• 2026: March 8 to March 28, and October 25 to October 31
• 2027: March 14 to March 27, and October 31 to November 6

These dates were computed from the IANA time zone database. A recurring transatlantic call fixed in UTC will appear an hour earlier or later on one side during these weeks. For the US side of the rules, see EST vs EDT and PST vs PDT.

Converting UTC/GMT to US time

What the "Z" in timestamps means

You'll often see times written like 2026-10-16T14:30:00Z. The trailing Z means the time is in UTC. RFC 3339, the internet standard for date-time formats, defines "Z" as a suffix that denotes a UTC offset of 00:00, often spoken as "Zulu" from the ICAO phonetic alphabet. That's where the term "Zulu time" comes from.

• 2026-10-16T14:30:00Z and 2026-10-16T14:30:00+00:00 mean the same instant.
• 2026-10-16T10:30:00-04:00 is also the same instant, written in Eastern Daylight Time.
• RFC 3339 gives -00:00 a special meaning: the time is known in UTC, but the local offset is unknown.
• A leap second appears as 23:59:60Z, which RFC 3339 allows at the end of a month that has one.

Unix timestamps take a different approach. They count seconds since 1970-01-01 00:00:00 UTC and treat every day as exactly 86,400 seconds, so a leap second doesn't get its own number. Unix time explained covers how that works, and the timestamp converter turns any epoch value into a readable UTC date.

Which should you use?

• Use UTC for anything technical or international: server logs, databases, APIs, flight plans, and meetings across several countries. It never shifts for daylight saving.
• Use GMT only when you mean the UK's winter clock time, or when copying a source that uses it. Treat "GMT" in older documents as UTC; the sub-second difference doesn't matter outside astronomy.
• Use "UK time" or "London time" when you mean whatever clocks in London show on that date, BST or GMT.

For more developer-side conversions, browser-based developer tools lists other utilities worth keeping handy.

Frequently asked questions

Is UTC the same as GMT?

For everyday clock time, yes: 12:00 UTC is 12:00 GMT. Technically, UTC is an atomic time standard and GMT is a rotation-based time, but leap seconds keep UTC within 0.9 seconds of astronomical time.

Does UTC change for daylight saving time?

No. UTC never changes for daylight saving time. Time zones are defined as offsets from UTC, and it is the zones, such as Eastern or UK time, that shift.

Is London on GMT all year?

No. UK clocks go forward one hour at 1:00 a.m. on the last Sunday in March to British Summer Time (UTC+1) and return to GMT at 2:00 a.m. on the last Sunday in October.

What does Z mean at the end of a timestamp?

Z means the time is in UTC, with an offset of 00:00. RFC 3339 defines it this way and notes it is often spoken as Zulu, from the ICAO phonetic alphabet.

When was the last leap second?

The most recent leap second was added at the end of December 31, 2016. NIST's table lists 27 leap seconds since 1972, and UTC is currently 37 seconds behind International Atomic Time.

Related tools

• UTC to EST Converter
• GMT to EST Converter
• Unix Timestamp Converter
• Time Zone Converter