GMT Offsets to Every Time Zone

A clock in Abidjan reads 14:00. A clock in New York reads 09:00. The difference is five hours, and that difference has a name: the GMT offset. It is not always a whole number, it is not always intuitive, and the sign convention in the IANA database is the opposite of what most people expect. But the system itself is straightforward once you strip away the abbreviations and the daylight saving noise.

How a GMT offset works

An offset is the number of hours and minutes a location sits ahead of or behind Greenwich Mean Time. GMT itself is fixed at UTC+0, the reference line at the Royal Observatory in Greenwich. A location at GMT+3 runs three hours ahead of that line. A location at GMT-5 runs five hours behind.

The only number you need

The offset is the single number you need. Know a city's offset, and you can calculate its time relative to GMT without knowing its time zone name, its DST rules, or its political boundaries.

Why the reference line matters

The prime meridian was established at the International Meridian Conference in Washington DC in October 1884. Before that, towns kept their own local mean time. Bristol ran 10 minutes behind London. Railway companies forced standardisation: the Great Western Railway adopted "London Time" in 1840, and by 1880 the Statutes (Definition of Time) Act made GMT the legal time for Great Britain.

Positive and negative offsets

Positive offsets mean "ahead of Greenwich." Negative offsets mean "behind." The sign is strict: GMT+5 is five hours later than GMT, never earlier.

A worked example across three cities

New York sits at GMT-5 on 15 January 2026. London sits at GMT+0. London is five hours ahead. Noon in New York is 17:00 in London. Tokyo runs at GMT+9 on the same date. Noon in London is 21:00 in Tokyo.

The 26-hour spread

The date flips at the International Date Line. GMT+14, used by Kiritimati in the Line Islands, is the first inhabited place to start a new day. GMT-12, used by uninhabited Baker Island and Howland Island, is the last. When it is noon on Thursday in London, it is 02:00 on Friday in Kiritimati and 00:00 on Thursday at Baker Island. That is a 26-hour spread between the two extremes of the same calendar date.

The full range: GMT-12 to GMT+14

The complete span runs 26 hours. Most of the world uses whole-hour steps, but the extremes exist and are in active use.

No time zone on Earth falls outside this range.

Half-hour and 45-minute offsets

Several countries reject the whole-hour convention. Their offsets are deliberate, not errors.

30-minute steps

India adopted GMT+5:30 in 1906 to split the difference across the subcontinent. Sri Lanka follows the same offset. Myanmar uses GMT+6:30. Iran runs at GMT+3:30. Afghanistan sits at GMT+4:30. Australia uses GMT+9:30 in the Northern Territory and GMT+10:30 on Lord Howe Island.

45-minute steps

Nepal's GMT+5:45 places its meridian precisely between two standard lines. The Chatham Islands, part of New Zealand, use GMT+12:45 to align with local solar noon. When you see GMT+5:30 or GMT+5:45 in a schedule, it is not a typo.

Why an offset beats a time zone name

Time zone names are ambiguous by design. CET means GMT+1 in winter and GMT+2 in summer, when it becomes CEST. EST is GMT-5 in winter; EDT is GMT-4 in summer. Two people in different cities can say "EST" on the same day and mean different things depending on whether daylight saving is in force where they are.

The number does not shift

A GMT offset has no such ambiguity. GMT+1 means exactly one hour ahead of the zero meridian, every day of the year. It does not shift with the seasons, it does not depend on national policy, and it does not require you to know whether Paris is on CET or CEST. When precision matters, use the number.

Who relies on this

This is why aviation and maritime operations use GMT exclusively. "Zulu time" (the NATO term for GMT) is identical in every cockpit and every ship's bridge on Earth. A pilot crossing the Atlantic does not care what time New York thinks it is. She cares what time it is at the prime meridian, because that is what the flight plan requires. NATO forces, amateur radio operators, and server administrators all log in UTC for the same reason.

The Etc/GMT sign reversal

The IANA time zone database includes zones named Etc/GMT, Etc/GMT+1, Etc/GMT-1, and so on. The naming is a trap. Etc/GMT+5 does not mean GMT+5. It means UTC-5: five hours behind the prime meridian. The sign is reversed.

Where the reversal comes from

This is a POSIX compatibility artefact. POSIX time zone strings treat positive numbers as west of the zero meridian, the opposite of the civilian convention. So Etc/GMT-5 equals UTC+5, and Etc/GMT+5 equals UTC-5. Etc/GMT with no sign is the correct zone for UTC+0.

How to stay safe

If you are setting a server clock and you see Etc/GMT-8, that is UTC+8, not UTC-8. The safest path is to avoid Etc/GMT zones entirely unless you are maintaining a legacy POSIX system. Use UTC or a specific IANA zone such as Europe/London or Africa/Abidjan.

What is my GMT offset?

Check your device

Your phone or computer displays the current offset in its time settings, usually as "UTC+5" or "GMT-4."

Look up your IANA zone

Your operating system uses the IANA database. Find your city's zone (for example, Asia/Tokyo or America/New_York) and the database supplies the current offset.

Calculate from longitude

Every 15 degrees of longitude equals roughly one hour. A location at 75°W is about five hours behind the prime meridian (75 ÷ 15 = 5). This gives the solar offset. Political boundaries and DST can shift the legal offset by an hour or more.

GMT offsets and their major cities

What is the difference between GMT and UTC?

GMT is a time zone fixed at UTC+0. UTC is an atomic time standard maintained by the BIPM in Paris. On a clock face, they read identically. Technically, GMT can refer to UT1 (astronomical time), which drifts from UTC by up to 0.9 seconds. For scheduling a call or setting a server, the distinction is irrelevant. Many style guides recommend "UTC" for precision. "GMT" persists in UK law, aviation, maritime, and broadcasting.

Why does the UK switch between GMT and BST?

The UK uses GMT (UTC+0) in winter and British Summer Time (UTC+1) in summer. The Summer Time Act 1972 defines BST as one hour ahead of GMT. Clocks go forward on the last Sunday of March and back on the last Sunday of October. This is why "London time" is ambiguous: London is on GMT only from late October to late March. The EU voted to end seasonal clock changes in 2019, but implementation has stalled and no EU-wide decision had been reached as of 2026. The UK government has indicated no plans to change the current arrangement.

What is Zulu time?

Zulu time is the military and aviation term for GMT. The letter Z (Zulu in the NATO phonetic alphabet) designates the zero meridian. When a pilot says "16:00 Zulu," she means 16:00 GMT, regardless of her local time zone. All flight plans, NOTAMs, and air traffic control communications use Zulu time. A timestamp suffix of "Z" (as in 2025-01-20T01:56:25Z) means the same thing.

What is the largest GMT offset?

The largest inhabited offset is GMT+14, used by Kiritimati (Christmas Island) in Kiribati. The smallest is GMT-12, used by Baker Island and Howland Island, both uninhabited. The gap between them is 26 hours.

Why are Etc/GMT signs reversed?

The IANA Etc/GMT zones inherit POSIX's sign convention, where positive numbers mean west of the prime meridian. Etc/GMT-5 is UTC+5. Etc/GMT+5 is UTC-5. Etc/GMT with no sign is UTC+0. Avoid the signed variants unless you are working with a POSIX system that demands them.

How do I convert my local time to GMT?

Take your local time, find your offset, and subtract it. If you are at GMT+8 and your clock reads 14:00, subtract eight hours: 06:00 GMT. If you are at GMT-5 and your clock reads 14:00, add five hours: 19:00 GMT. If your location observes DST, check whether it is in force on the date you are converting.

Know your own offset. When you schedule across time zones, write "GMT" or "UTC", never "London time." London changes. The number does not.