The short answer
If a webinar says 15:00 UTC or 15:00 GMT, both mean the same moment. UTC is the reference: every time zone is defined as an offset from it, such as UTC-5 for New York in winter or UTC+3 for Istanbul. GMT is one of those zones, with an offset of zero, and it is what clocks in London show only in winter.
So 15:00 UTC is 15:00 in London in January but 16:00 in London in July. When you need a fixed reference that does not move with the seasons, say UTC. The converter below turns a UTC time into your local time, with daylight saving applied for the date you enter.
Try it here: UTC to Local Time
Open the full tool pageWhere GMT and UTC come from
Greenwich Mean Time is the mean solar time at the Royal Observatory in Greenwich, London. In 1884 an international conference chose the Greenwich meridian as longitude zero, and GMT became the starting point for the world's time zones. It was based on the rotation of the Earth, measured by astronomers.
The Earth does not rotate perfectly evenly, so in the 1960s timekeeping moved to atomic clocks. Coordinated Universal Time combines the two: it ticks with atomic seconds, and since 1972 leap seconds have been inserted occasionally to keep it within 0.9 seconds of the Earth's rotation. 27 leap seconds have been added so far, the last at the end of 2016, and the international body for weights and measures decided in 2022 to stop adding them by 2035.
Scientists distinguish UTC from the astronomical time scale behind GMT, but the gap is always under a second. Nobody scheduling a call needs to care about it. The name UTC, by the way, is a compromise between the English and French word orders, which is why the letters do not match either language.
Offsets, zones and daylight saving time
A UTC offset tells you how far a local clock is ahead of or behind UTC. Offsets run from UTC-12 to UTC+14, and not all are whole hours: India uses UTC+5:30, Nepal UTC+5:45 and parts of Australia UTC+9:30. A time zone such as Europe/Berlin is more than an offset: it is a set of rules that says which offset applies on which date, including daylight saving time and historical changes.
Daylight saving time moves clocks forward one hour in summer. The US and Canada switch on the second Sunday in March and back on the first Sunday in November; the EU and UK on the last Sundays of March and October. Many countries do not switch at all, including China, India, Japan, Turkey (UTC+3 all year since 2016) and most of Africa, and in the southern hemisphere summer time runs from about October to April. That is why the difference between two cities is not fixed, and why a good converter needs the date, not just the time.
EST, EDT, PST, PDT and other abbreviations
North American abbreviations come in pairs: S for standard time (winter) and D for daylight time (summer). People often write EST all year, which is technically wrong for about eight months. If you are not sure, write ET or PT, which mean whatever is in effect on that date, or give the UTC time.
| Abbreviation | Name | Offset | When |
|---|---|---|---|
| GMT | Greenwich Mean Time | UTC+0 | UK and Ireland in winter, Iceland and parts of West Africa all year |
| BST | British Summer Time | UTC+1 | UK, late March to late October |
| CET / CEST | Central European Time / Summer Time | UTC+1 / UTC+2 | Most of continental Europe |
| EST / EDT | Eastern Standard / Daylight Time | UTC-5 / UTC-4 | New York, Toronto, Miami |
| CST / CDT | Central Standard / Daylight Time | UTC-6 / UTC-5 | Chicago, Dallas |
| MST / MDT | Mountain Standard / Daylight Time | UTC-7 / UTC-6 | Denver; Arizona stays on MST all year |
| PST / PDT | Pacific Standard / Daylight Time | UTC-8 / UTC-7 | Los Angeles, Seattle, Vancouver |
| IST | India Standard Time | UTC+5:30 | India all year |
| JST | Japan Standard Time | UTC+9 | Japan all year |
How to write times for an international audience
A time without a zone is a guess for everyone outside your city. These habits remove almost all confusion:
- Give UTC as the anchor, then one or two local times: 15:00 UTC (10:00 New York, 16:00 London).
- Use 24 hour times in international text. If you use a.m. and p.m., write noon and midnight instead of 12:00 p.m. and 12:00 a.m.
- Name cities rather than abbreviations: CST is both US Central and China Standard Time, and IST stands for India, Ireland or Israel.
- Include the weekday and date. A Monday evening event in California is Tuesday morning in Asia.
- For machines and logs, use ISO 8601 with an offset or Z: 2026-11-03T15:00:00Z, where Z (Zulu) means UTC.
- Avoid scheduling recurring events in UTC for people who live by local time without telling them: their local time will shift by an hour when their daylight saving starts or ends.
Converting UTC to your local time
Server logs, API responses, flight plans, weather data and many online launches use UTC. To read them, add your offset: if you are in New York in winter (UTC-5), 14:00 UTC is 9:00 local time; in summer (UTC-4) it is 10:00. Crossing midnight changes the date, so 02:00 UTC on Tuesday is still Monday evening in the Americas.
The converter on this page does the arithmetic for you. Your local time zone is detected from your device, and the offset is taken from the time zone rules built into your browser for the exact date, including half hour zones. Add other cities to see the same moment for colleagues or customers, and use Convert from here on any card to start from that city instead.
Common mistakes
- Treating London time as GMT in summer: from late March to late October London is on BST, one hour ahead of UTC.
- Writing EST in July: New York is on EDT then, and a strict reader will be an hour off.
- Adding the offset the wrong way: UTC-5 means local time is 5 hours behind UTC, so you subtract 5 from the UTC time to get local time.
- Forgetting the date change near midnight when converting log entries or flight times.
- Assuming the difference between two cities is constant: it changes for a few weeks each spring and autumn when regions switch on different dates.