UTC, GMT, and Local Time for International Meetings

UTC stands for Coordinated Universal Time (the acronym is a compromise between the English CUT and the French TUC). It is the primary time standard by which the world regulates clocks and time. Unlike time zones — which shift by region and change with DST — UTC never changes. It ticks forward at the same rate, day and night, summer and winter, everywhere on Earth.

UTC is often confused with GMT (Greenwich Mean Time). They share the same zero meridian — the prime meridian running through Greenwich, London — and for everyday purposes are effectively interchangeable. The technical difference is that GMT is an astronomical time zone, while UTC is an atomic standard maintained by a network of atomic clocks worldwide. Be careful with GMT in invitations: the UK uses GMT only in winter and British Summer Time (UTC+1) for about seven months of the year, so London time and GMT differ for much of it. Modern computing uses UTC, not GMT.

Servers, databases, and APIs store timestamps in UTC to avoid ambiguity. When you post a message, book a flight, or send an email, the underlying timestamp is UTC. Your device converts it to your local time for display. This is why changing your phone's timezone doesn't reorder your messages — the underlying UTC timestamp is unchanged.

Aviation uses UTC (called 'Zulu time' in pilot parlance — Z is the suffix for UTC) for all flight plans, air traffic control communications, and weather reports. Global financial markets also use UTC-based timestamps for trade records, ensuring a single unambiguous timeline across exchanges in different time zones.

Some distributed teams use UTC as their scheduling anchor: all meeting invites are expressed as a UTC time and each person converts to their local time themselves. This works well when your team is comfortable with UTC offsets and you want to avoid DST confusion entirely. The downside is that it requires everyone to know their current offset, which changes with DST.

UTC is built from hundreds of atomic clocks in laboratories around the world, which are combined into International Atomic Time. Because the Earth's rotation is slightly irregular and gradually slowing, atomic time slowly drifts away from solar time. To keep UTC within a second of the Earth's rotation, timekeepers have occasionally inserted a leap second at the end of June or December. Twenty-seven leap seconds were added between 1972 and 2016, and none have been added since. In 2022 the international body responsible for measurement standards voted to stop using leap seconds by or before 2035, which will let UTC run continuously and simplify software. For everyday scheduling leap seconds are invisible; they matter mostly to engineers who build systems that need sub-second accuracy.

You will often see times written like 2026-03-15T14:00:00Z. The date comes first as year, month, and day; the letter T separates the date from the time; and the Z at the end means Zulu time, which is UTC. A timestamp such as 2026-03-15T19:30:00+05:30 describes the same instant expressed in India Standard Time, because +05:30 is the local offset from UTC. Once you know this pattern you can read timestamps in logs, calendar files, APIs, and emails without guessing. Both examples above describe the same moment: 14:00 UTC is 19:30 in Mumbai.

To convert from UTC to local time, add the local offset. To convert from local time to UTC, subtract it.

• 14:00 UTC in Mumbai (UTC+5:30) is 19:30.
• 14:00 UTC in Tokyo (UTC+9) is 23:00.
• 14:00 UTC in Los Angeles is 06:00 in winter (UTC-8) and 07:00 in summer (UTC-7).
• 9:00 AM in Berlin is 08:00 UTC in winter (UTC+1) and 07:00 UTC in summer (UTC+2).

The last two examples show why the same city needs two different conversions during the year, and why a named time zone is more reliable than a remembered number.

Offsets run from UTC-12 to UTC+14, a spread of 26 hours. UTC+14 is used by the Line Islands of Kiribati, which are among the first places on Earth to begin each new calendar day. UTC-12 is a nominal zone covering uninhabited islands, but it has a practical use: deadlines written as 'Anywhere on Earth' or AoE use it, meaning the deadline passes only when the date has ended everywhere on the planet. If you see AoE on a submission deadline, treat it as 12:00 UTC on the following day.

UTC is ideal for recording when something happened and for coordinating across regions. It is the wrong tool for anything defined by local wall-clock time, such as a shop that opens at 9 AM every day or a weekly class that starts at 10 AM local time. If you store those as a fixed UTC instant, they drift by an hour whenever daylight saving changes. For recurring local events, keep the local time together with a named time zone. For one-time global events, keep the UTC instant.

The same UTC time gives different local times in winter and summer for cities that change their clocks. Hong Kong and Mumbai do not change, so their local time stays the same.

Write meeting times as a local time in a named zone plus the UTC time, for example 'Tuesday 13 July 2027, 15:00 Europe/London (14:00 UTC)'. Anyone with a wrong device setting can still check the UTC value.

Times are computed in your browser from the IANA time zone database for the dates shown.