UTC and GMT show the same clock time, but they aren’t the same thing. UTC (Coordinated Universal Time) is the world’s time standard, kept by atomic clocks. GMT (Greenwich Mean Time) is the time zone the UK uses in winter; in summer the UK moves to BST, one hour ahead of UTC.
For scheduling a call, the practical difference is small: 15:00 UTC and 15:00 GMT are the same moment. The confusion comes from the UK’s summer time, from old usage of GMT as a scientific standard, and from labels like “Zulu” and “Z”. This guide explains each term, the UK’s clock changes, how to convert UTC to US time, and which one to use when you write a time down.
What UTC is
UTC is the reference that every civil time zone is defined from. According to NIST, it is based on International Atomic Time (TAI), which is computed from hundreds of atomic clocks around the world, and it is adjusted by leap seconds to account for the difference between the atomic second and the Earth’s rotation. The official UTC is calculated by the International Bureau of Weights and Measures (BIPM) near Paris; national labs such as NIST keep their own real-time versions of it.
UTC never changes for summer or winter. It has no daylight saving time, which is why pilots, programmers, scientists and the military use it to avoid ambiguity.
What GMT is
GMT has two meanings. Originally it was mean solar time at the Royal Observatory in Greenwich, London, on the prime meridian, and it served as the world’s time reference before UTC. NIST notes that today GMT is more commonly used for the time zone at the prime meridian, as a local representation of UTC.
In everyday use, then, GMT means “the time in London in winter”, and it reads the same as UTC. The UK’s official advice from GOV.UK is that when the clocks go back, the UK is on Greenwich Mean Time.
UTC vs GMT: the practical difference
| UTC | GMT | |
|---|---|---|
| What it is | A time standard | A time zone (and, historically, a solar time) |
| Based on | Atomic clocks, with leap seconds | The prime meridian at Greenwich; today set to match UTC |
| Changes in summer? | Never | The UK leaves GMT for BST (UTC+1) in summer |
| Used by | Aviation, computing, science, international scheduling | The UK in winter, in everyday speech |
| Clock reading | The reference | Identical to UTC whenever a place is on GMT |
The trap is summer. If a British colleague says “3 p.m. GMT” in July, they almost always mean 3 p.m. London time, which is 3 p.m. BST, or 14:00 UTC. If precision matters, ask, or write the time in UTC.
British Summer Time and the UK clock changes
GOV.UK states the rule: 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. The period in between is British Summer Time (BST), UTC+1.
- Sunday, October 25, 2026: the UK returns to GMT.
- Sunday, March 28, 2027: the UK moves to BST.
- Sunday, October 31, 2027: the UK returns to GMT.
The US changes on different weekends, so the gap between London and New York shrinks to four hours for a week or two each spring and fall. The daylight saving time guide lists those weeks, and the GMT to Eastern time chart shows the hour-by-hour conversion.
Converting UTC to US time zones
Subtract the zone’s offset from UTC. US zones are 5 to 10 hours behind UTC in winter, and the zones that observe daylight time are one hour less behind in summer. Here is what 12:00 UTC (noon) looks like across the country:
| Place | Winter (e.g. January 15, 2027) | Summer (e.g. July 15, 2027) |
|---|---|---|
| London | 12:00 p.m. GMT | 1:00 p.m. BST |
| New York (Eastern) | 7:00 a.m. EST (UTC−5) | 8:00 a.m. EDT (UTC−4) |
| Chicago (Central) | 6:00 a.m. CST (UTC−6) | 7:00 a.m. CDT (UTC−5) |
| Denver (Mountain) | 5:00 a.m. MST (UTC−7) | 6:00 a.m. MDT (UTC−6) |
| Phoenix (Arizona) | 5:00 a.m. MST (UTC−7) | 5:00 a.m. MST (UTC−7) |
| Los Angeles (Pacific) | 4:00 a.m. PST (UTC−8) | 5:00 a.m. PDT (UTC−7) |
| Anchorage (Alaska) | 3:00 a.m. AKST (UTC−9) | 4:00 a.m. AKDT (UTC−8) |
| Honolulu (Hawaii) | 2:00 a.m. HST (UTC−10) | 2:00 a.m. HST (UTC−10) |
For a specific date and time, use the UTC to Eastern time converter or the full time zone converter, which applies the right offset for each date. The EST vs EDT guide lists every US zone and its abbreviations.
Leap seconds, Zulu time and GPS time
- Leap seconds: NIST says leap seconds keep UTC within 0.9 seconds of astronomical time (UT1). The first was added on June 30, 1972, and the most recent on December 31, 2016. TAI is now 37 seconds ahead of UTC.
- Zulu time: the “Z” after a time, as in 14:00Z, means UTC. NIST notes that the Z zone is equivalent to UTC and that aviation and the military call it Zulu. Our military time chart covers reading those four-digit times.
- GPS time: GPS doesn’t apply leap seconds, so it runs ahead of UTC by the number of leap seconds added since January 6, 1980. Receivers correct for this before showing you the time.
Which one should you write?
- International meetings and deadlines: write UTC. It never shifts, so a time in UTC means the same thing in March and in October.
- Talking to people in the UK: say “London time”, or write GMT in winter and BST in summer.
- Software and data: store timestamps in UTC, for example in ISO 8601 form such as 2026-10-02T14:00:00Z, and convert to local time only for display.
To find what the clock will read a set number of hours from now, in your own time zone, use the hours from now calculator.
Frequently asked questions
Is UTC the same as GMT?
They show the same time, so for everyday scheduling you can treat them as equal. Technically UTC is the atomic time standard and GMT is a time zone, and the UK leaves GMT for BST in summer, while UTC never changes.
Is London on GMT or BST right now?
London uses BST (UTC+1) from the last Sunday in March to the last Sunday in October, and GMT the rest of the year. In 2026 it switches back to GMT on Sunday, October 25.
How many hours ahead is UTC from Eastern time?
Five hours when Eastern time is on EST in winter, and four hours when it’s on EDT in summer.
Does UTC change for daylight saving time?
No. UTC stays the same all year. Local time zones move relative to it when they switch between standard and daylight time.
What does the Z at the end of a time mean?
Z stands for the zero-offset zone, which is UTC, and is read as “Zulu”. 2026-10-02T14:00:00Z is 2:00 p.m. UTC on October 2, 2026, which is 10:00 a.m. EDT in New York.