UTC, GMT and the prime meridian
Two abbreviations appear constantly when world times are discussed: UTC and GMT. They look alike but are not the same. Once you understand the difference, you understand how the entire time zone system fits together.
GMT — time measured by the sky
GMT stands for Greenwich Mean Time — the mean solar time as observed from the Royal Observatory in Greenwich, London. The observatory was founded in 1675 to make precise astronomical time observations, partly for navigation at sea. Sailors could base their calculation of longitude on Greenwich as a fixed reference point.
At the International Meridian Conference of 1884 in Washington, 26 countries agreed on Greenwich as the 0° meridian — the prime meridian — along which the world was divided into time zones. GMT became the time standard for most countries. That prime meridian passes through Greenwich, runs through Europe and Africa, and ends at the South Pole.
UTC — time measured by atomic clocks
In the twentieth century it was discovered that Earth's rotation is not perfectly constant: it sometimes speeds up, sometimes slows down slightly. Science and technology needed a more stable standard. In 1967 the international community defined the second based on the oscillation frequency of the caesium-133 atom: exactly 9,192,631,770 oscillations per second. On that foundation UTC — Coordinated Universal Time — was built.
UTC is maintained by an international network of more than 400 atomic clocks in approximately 85 laboratories, coordinated by the Bureau International des Poids et Mesures (BIPM) in Paris. To prevent UTC from drifting too far from astronomical time (based on Earth's rotation), leap seconds are occasionally inserted — always on 30 June or 31 December. Between 1972 and 2023, 27 leap seconds were added (source: Royal Observatory Greenwich).
GMT and UTC are interchangeable in everyday practice: they never differ by more than 0.9 seconds. However, UTC is the international scientific norm, while GMT is formally a time zone (UTC±0 in winter). Back to the overview: time zones explained.
The prime meridian: 0° longitude
The 0° meridian runs from the North Pole through Greenwich, through the east of Europe and West Africa, to the South Pole. If you ever visit the observatory in Greenwich, you can stand with one foot in the western and one foot in the eastern hemisphere.
How to read a UTC offset
A UTC offset tells you how many hours (and sometimes minutes) the local time is ahead of or behind UTC. Read the offset as follows:
Step 1 — Find the offset of the location
Every time zone has a fixed offset, such as UTC+2 for Central European Summer Time or UTC−5 for Eastern Standard Time on the US East Coast. The IANA Time Zone Database lists all valid offsets.
Step 2 — Add or subtract from UTC
If the offset is positive (+), add the number to UTC. If the offset is negative (−), subtract it from UTC. If it is 10:00 UTC and the offset is UTC+9, the local time is 19:00. If the offset is UTC−5, it is 05:00.
Step 3 — Watch out for half- and quarter-hours
Zones such as India (UTC+5:30) and Nepal (UTC+5:45) include minutes. Add the minutes to the local time too. If it is 10:00 UTC, it is 15:30 in India and 15:45 in Nepal.
Example
It is 08:00 UTC. What is the time in the following cities?
• Amsterdam (UTC+2 summer time): 10:00
• New York (UTC−4 summer time): 04:00
• New Delhi (UTC+5:30, no daylight saving): 13:30
• Kathmandu (UTC+5:45, no daylight saving): 13:45
• Tokyo (UTC+9, no daylight saving): 17:00
UTC and the Date Line
If you travel from the 0° meridian in one direction all the way around the world, you eventually reach the 180° meridian in the Pacific Ocean — roughly the counterpart of Greenwich. There lies the International Date Line, where the date changes. Crossing westward means a day forward; eastward a day back. Everything about that line is on the page the International Date Line.
For the broad overview of how time zones interact across continents, see time zones explained. Want to know how latitudes and the equator divide the Earth? Read the equator and hemispheres.
Sources
- Royal Observatory Greenwich — history of GMT and the prime meridian (rmg.co.uk)
- IANA Time Zone Database — tz.iana.org (all UTC offsets and time zone names)
- Bureau International des Poids et Mesures (BIPM) — bipm.org (definition of UTC and the SI second)
- International Meridian Conference, Washington 1884 — establishment of the 0° meridian