← All blog posts · 4 November 2026

UTC vs GMT — What's the Difference?

People write "GMT+5" and "UTC+5" as if the two abbreviations were interchangeable — and for booking a flight, they effectively are. But UTC and GMT are not the same thing. One is the world's official time standard, measured by atomic clocks. The other is a time zone — the local time used in the United Kingdom in winter. Here is the difference, why both survive, and which one you should use.

GMT: the original zero

GMT stands for Greenwich Mean Time. It began as the average solar time at the Royal Observatory in Greenwich, London — "mean" because it averages out the small daily wobbles in the sun's apparent motion. In 1884, an international conference made the Greenwich meridian the Prime Meridian of the world, longitude zero, and GMT became the reference from which every other zone was counted. For most of a century, GMT was world time: radio time signals, shipping and aviation all ran on it.

Today GMT survives as a time zone name. The UK observes GMT in winter and moves to British Summer Time (UTC+1) in summer, which is why London's offset reads UTC+0 in January and UTC+1 in July. A few other places, such as Iceland, keep GMT all year. As a zone, GMT is a rule about what clocks on a wall should show — nothing more.

UTC: the atomic standard

UTC — Coordinated Universal Time — was introduced in the 1960s and became the world's standard in 1972. Instead of being read from the sun, UTC is computed from hundreds of atomic clocks in laboratories around the world, making it far more stable than Earth's rotation, which speeds up and slows down slightly and is gradually slowing overall.

To stop UTC drifting away from solar time, scientists occasionally insert a leap second — an extra second added to a day, most recently on 31 December 2016 — keeping UTC within 0.9 seconds of the Earth-based time it replaced. (An international decision in 2022 set out to stop adding leap seconds by 2035, letting the two drift apart rather than keep disrupting computer systems.) Every time zone on Earth is now defined as an offset from UTC, and the systems that run aviation, finance, GPS and the internet all synchronise to it. The UTC clock at the top of our homepage shows it ticking, live.

So which one is Pakistan?

Pakistan Standard Time (PKT) is officially UTC+5: five hours ahead of UTC, all year, with no daylight saving — see the live offset on the Karachi page. You will still see "GMT+5" on older websites and some timetables; it refers to exactly the same five hours, counted from the older reference. The difference between the notations is bookkeeping, not time: a meeting at 15:00 PKT is 10:00 UTC (or "10:00 GMT") either way. Our converter works in UTC offsets throughout, which is the modern, unambiguous system.

The practical rule

Use UTC when you are describing an offset or anything technical — UTC+5, UTC−4 — because that is how zones are officially defined today, and how the IANA database behind this site expresses them. Expect to keep seeing GMT in British contexts, where it names London's winter time zone. And if the two ever seem to disagree by a second, that is the leap-second system doing its job. For the bigger picture of how all these zones fit together, read our guide to how time zones work.