What Is UTC? Coordinated Universal Time Explained

UTC — Coordinated Universal Time — is the primary time standard by which the world regulates clocks and time. It is the reference point that every time zone on Earth is defined against. When you see a time written as "14:00 UTC", it means the same instant everywhere on the planet, regardless of local clocks. You can see the current value any time on our UTC clock.

Where UTC comes from

UTC is calculated from two ingredients. The first is International Atomic Time (TAI), an extraordinarily precise timescale averaged from hundreds of atomic clocks in laboratories around the world. Atomic clocks measure the vibrations of caesium atoms and drift by less than a second over millions of years.

The second ingredient is the actual rotation of the Earth, known as UT1. Because the planet's spin is slightly irregular and gradually slowing, atomic time and astronomical time drift apart. To keep them aligned, timekeepers occasionally insert a leap second into UTC — a one-second adjustment that stops UTC from wandering more than 0.9 seconds away from the Sun's position in the sky.

UTC and time zones

Every time zone is expressed as an offset from UTC. London in winter is UTC+0, New York is UTC−5, Tokyo is UTC+9, and India is UTC+5:30. To find a local time, you simply add or subtract the offset:

• 16:00 UTC + 9 hours = 01:00 the next day in Tokyo
• 16:00 UTC − 5 hours = 11:00 the same day in New York

Because everyone agrees on UTC as the anchor, converting between any two places is just arithmetic on their offsets. Our time zone converter does this for you, and the meeting planner uses the same idea to find overlapping working hours across countries.

Why the world needs a single standard

Before universal time, every town kept its own clock based on local noon, which made railway timetables and later telegraph and aviation schedules a nightmare. A shared standard lets a pilot in London, an engineer in Sydney and a server in a data centre all agree on exactly when something happened. UTC is used by aviation, the internet, financial markets, GPS and scientific research precisely because it removes any ambiguity.

UTC in computing

Almost every server and database stores timestamps in UTC and only converts to local time when displaying them to a user. This avoids bugs when clocks change for daylight saving or when users are spread across the globe. If you have ever seen a timestamp with a trailing "Z" (for example 2026-02-10T16:00:00Z), the "Z" — pronounced "Zulu" — means the time is in UTC. Related to this is the Unix timestamp, which counts seconds from a fixed UTC starting point.

UTC vs GMT

People often use UTC and GMT interchangeably, and in casual conversation the difference rarely matters. But they are not quite the same thing: GMT is a time zone, while UTC is a technical standard. We cover the distinction in GMT vs UTC.