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For everyday civil use, Greenwich Mean Time (GMT) and Coordinated Universal Time (UTC) show the same clock time — both are UTC+0 — so a timestamp labelled GMT and the same timestamp labelled UTC name the same instant, to within a fraction of a second. They differ in kind: UTC is a global time standard kept by atomic clocks and nudged by leap seconds to stay within 0.9 seconds of the Earth's rotation, while GMT is a time zone — the United Kingdom's winter time — and, historically, an astronomical timescale read from the Sun over Greenwich. The two coincide only in winter: from late March to late October the United Kingdom moves its clocks forward to British Summer Time (UTC+1), so London is then an hour ahead of GMT.

It depends what you count: geometry divides the globe into 24 zones of 15° longitude each, but civil clocks use about 38 distinct offsets from Coordinated Universal Time (UTC) — spanning UTC−12 to UTC+14 — because several regions sit on half-hour or 45-minute offsets, and the IANA database that software relies on lists several hundred named zones because it records each region's full history of change.

When the clocks change depends on the country. The United States springs forward on the second Sunday of March and back on the first Sunday of November; the European Union and the United Kingdom switch on the last Sunday of March and the last Sunday of October; and Australia's daylight-saving states — in the southern hemisphere, so on the opposite calendar — go forward on the first Sunday of October and back on the first Sunday of April.

EST (Eastern Standard Time) is the winter setting of US Eastern Time, five hours behind Coordinated Universal Time (UTC−5); EDT (Eastern Daylight Time) is the summer setting, four hours behind (UTC−4). 'Eastern Time' (ET) is the umbrella that switches between them — EDT from the second Sunday in March to the first Sunday in November, EST the rest of the year.

PST (Pacific Standard Time) is the winter clock on the North American Pacific coast, set eight hours behind Coordinated Universal Time (UTC−8); PDT (Pacific Daylight Time) is the summer clock, one hour ahead at UTC−7. Pacific Time, written PT, is the umbrella label that follows whichever is in force — switching to PDT on the second Sunday in March and back to PST on the first Sunday in November.

Most of the world does not use daylight saving time. Moving the clocks twice a year is mainly a North American, European, and Australasian practice; the great majority of countries — across most of Africa and almost all of Asia, and throughout the tropics — keep one standard time all year round. The habit also brings little benefit near the equator, where day length barely changes between seasons, and several countries that once kept it have since abolished it: Russia has stayed on permanent standard time since 2014, Turkey on permanent summer time since 2016, and most of Mexico dropped it at the end of 2022.

Zulu time is Coordinated Universal Time (UTC), the time at the zero-degree meridian, written with a trailing letter Z — so a time given as 1430Z means 14:30 UTC. The name is the phonetic-alphabet word for the letter Z, the designation the military and aviation give the zero-offset time zone, and it is the shared clock of aviation, military operations, shipping, and weather forecasting, where running everything on a single worldwide time removes the errors of converting between local zones.

To schedule a meeting across time zones, pick a slot that falls inside everyone's working hours, anchor that single moment to Coordinated Universal Time (UTC) — the global reference every local clock is offset from — and convert it once into each participant's local time. The one recurring trap is daylight saving time: a region's offset from UTC can change between the day you schedule and the day you meet, so an arrangement that looks right today can be an hour wrong on the date itself.

A time zone abbreviation is a short letter code for a region's civil time — EST, GMT, IST — but the codes are not standardised and many are ambiguous: IST alone can stand for India, Israel, or Irish time, and CST means both US Central Time and China Standard Time. For anything that has to be precise, identify a zone by its UTC offset (such as UTC−5) or its IANA name (such as America/New_York) rather than by its abbreviation.

CST (Central Standard Time) is the winter setting of US Central Time, six hours behind Coordinated Universal Time (UTC−6); CDT (Central Daylight Time) is the summer setting, five hours behind (UTC−5). Central Time, written CT, is the umbrella label that follows whichever is in force — switching to CDT on the second Sunday in March and back to CST on the first Sunday in November.