The world has one time — Coordinated Universal Time, UTC — and roughly forty local versions of it, each a fixed offset from UTC, set by law, and changed by daylight saving on schedules that differ by country and hemisphere. The system is a 19th-century solution to a railway problem that became a 21st-century scheduling problem: every meeting across zones is an exercise in converting twice and trusting neither clock face. This guide explains the offsets, the daylight-saving transitions that move them, the date line that breaks the day, and the practices that keep cross-zone scheduling correct.

UTC and offsets: the one time, and the local versions

Coordinated Universal Time is the reference: the time at 0° longitude, kept by atomic clocks, with no daylight saving and no seasonal change. Every time zone is an offset from UTC — UTC+1, UTC−5, UTC+5:30 — and the offset is set by the government of the territory, not by geography. China, spanning five geographic zones, uses one: UTC+8 everywhere, so the sun rises at what the clock calls mid-morning in the west. Spain, on UTC+1, sits geographically in the UTC+0 band. India uses UTC+5:30 and Nepal UTC+5:45 — the half- and quarter-hour offsets are reminders that the system is legal, not astronomical, and that the map’s neat 15° bands are a simplification the real world declined.

Converting is addition and subtraction on the offset: local time = UTC + offset. 15:00 UTC is 16:00 in Berlin (UTC+1), 10:00 in New York (UTC−5), and 20:30 in Mumbai (UTC+5:30). The offset is the whole conversion, and it is the quantity to check — the zone’s name (CET, EST, IST) is ambiguous (IST is India, Ireland, and Israel at different offsets), while the offset is not. State the offset, not the abbreviation, whenever a time must be understood in another place.

Daylight saving: the twice-yearly move, on different dates

Daylight saving time (DST) moves the clocks forward one hour in spring and back one hour in autumn, changing the offset by one hour for the summer months. The United States moves on the second Sunday in March and the first Sunday in November; the European Union moves on the last Sundays in March and October; the southern hemisphere — Australia, New Zealand, Chile — moves in the opposite direction, forward in October and back in April. The consequence is that the difference between two cities is not constant through the year: London and New York differ by 5 hours in winter and 4 in summer, and for the two or three weeks when one side has changed and the other has not, the difference is briefly 4 hours in the other direction of expectation — a window in which standing meetings shift by an hour and the shift is nobody’s error but the calendar’s.

The transitions themselves create two anomalies worth knowing. In spring, an hour does not exist: 02:00 to 03:00 is skipped, and a meeting scheduled in the gap has no time to occur in. In autumn, an hour occurs twice: 01:00 to 02:00 happens twice, and a timestamp in that hour is ambiguous without stating which occurrence. Systems that store times in UTC and convert for display avoid both anomalies; systems that store local times inherit them, and the autumn ambiguity is the one that corrupts logs, schedules, and billing records most quietly.

The International Date Line: a day apart, an hour close

The International Date Line runs roughly along 180° longitude, and crossing it changes the date by a day while the clock barely moves. Samoa and American Samoa, 24 km apart across the line, are a full day apart — and only an hour apart on the clock, since Samoa is UTC+13 and American Samoa UTC−11, a 24-hour difference that is also, paradoxically, a 1-hour difference in the other direction. Tonga (UTC+13) and Honolulu (UTC−10) differ by 23 hours, so a Monday morning call in one is a Sunday morning call in the other. The date line is a legal convention, not a physical feature, and it zigzags around island groups — Kiribati moved the line eastward in 1995 so the whole country shares one date — which is why the offset tables, not the longitude, are the authority.

For scheduling, the date line means the day must be stated with the time, always: “Monday 09:00” is not a complete specification across the line, and the conversion through UTC is what settles which Monday is meant. The time-zone calculation on this site carries the date with the time for exactly this reason, and a meeting invitation that omits the date is an invitation to a missed meeting.

Scheduling across zones: the practices that work

Four practices keep cross-zone scheduling correct. State the zone with every time: “15:00 UTC” or “15:00 London (UTC+1)”, never a bare clock face and never an ambiguous abbreviation. Put the meeting in the calendar in UTC: calendar software converts UTC to each attendee’s local time correctly, including across daylight-saving transitions; a time typed as local text does not convert at all. Check the transition windows: the weeks when the two sides change on different dates are when a standing meeting’s local time shifts, and the shift should be announced, not discovered. Rotate the burden: a meeting fixed at one UTC time is always at the same local inconvenience for the same people — rotating the UTC time across the week shares the early mornings and late nights, and the rotation should be explicit, not left to whoever complains first.

For recurring meetings, store the recurrence rule in one zone’s terms and let the calendar convert: a meeting at 09:00 New York time, recurring weekly, moves its UTC time when New York changes offset, and every other attendee’s local time moves with it. A meeting stored as a fixed UTC time does the opposite — it stays fixed in UTC and moves in every local time. The two are different meetings, and choosing the wrong one is how a weekly call silently migrates an hour twice a year.

A short history: railways, standard time, and the 40 zones

Before standard time, every town kept local solar time, and a railway timetable was a negotiation between dozens of local noons. Britain adopted railway time — Greenwich Mean Time across the network — in the 1840s, and the 1884 International Meridian Conference fixed Greenwich as the prime meridian. The United States divided itself into four zones in 1883, at the railways’ initiative, and Congress ratified the system in 1918 with the Standard Time Act, which also introduced daylight saving. The modern system has about 38–40 distinct offsets in use, from UTC−12 to UTC+14, including the half- and quarter-hour offsets and the zones that observe daylight saving on their own schedules — a legal patchwork over an astronomical grid, and the reason the offset tables must be consulted rather than derived.

Frequently asked questions

What is UTC?

Coordinated Universal Time, the world’s time reference, kept by atomic clocks at 0° longitude. It never observes daylight saving and never changes. Every time zone is an offset from UTC — UTC+1, UTC−5, UTC+5:30 — and converting between zones is arithmetic on the offsets, through UTC.

Why does the time difference between two cities change during the year?

Because daylight saving moves the two cities’ offsets independently, on dates that differ by country. London and New York differ by 5 hours in winter and 4 in summer, and for the two or three weeks when one side has changed and the other has not, the difference is briefly different again. Convert through UTC with the offsets in force on the actual date, not the offsets you remember.

What happens to the clock when daylight saving starts and ends?

In spring the clocks go forward one hour and an hour does not exist — 02:00 to 03:00 is skipped. In autumn the clocks go back and an hour occurs twice — 01:00 to 02:00 happens twice, and a timestamp in that hour is ambiguous without stating which occurrence. Systems that store times in UTC avoid both anomalies.

What is the International Date Line?

The roughly 180°-longitude line where the calendar date changes by a day. Crossing it eastward subtracts a day, westward adds one. Places either side can be a day apart while their clock times are close — Samoa (UTC+13) and American Samoa (UTC−11) are 24 hours apart on the calendar and an hour apart on the clock. The line zigzags around island groups, so the offset tables are the authority, not the longitude.

How should I state a time for people in other time zones?

With the zone and the offset, and the date: “Monday 15:00 UTC” or “Monday 15:00 London (UTC+1)”. Avoid bare clock faces and ambiguous abbreviations — IST is India, Ireland and Israel at different offsets. Put the meeting in the calendar in UTC so each attendee’s software converts to local time correctly.

Does every country observe daylight saving?

No. Most countries near the equator do not, because their day length barely varies. China, India and Japan do not observe it. The southern hemisphere observes it in the opposite season — forward in October, back in April. The European Union and the United States change on different weekends, which is what makes the difference between their cities vary through the year.