How time zones work

The time on any clock in the world comes from one simple sum: UTC plus the place's offset. Three things decide what that offset is:

• The offset itself: how many hours and minutes the place is ahead of or behind UTC.
• The boundary: which side of a time zone line the place falls on.
• The local rules: whether the place uses daylight saving time, and any changes its government has made.

If you're new to the idea of time zones, start with what is a time zone? This guide looks at how each of those three parts works.

UTC, or Coordinated Universal Time, is the reference every time zone is measured from. It's calculated from atomic clocks in laboratories around the world, and every so often a leap second has been added to keep it in step with the Earth's rotation.^[1]

UTC itself never changes with the seasons. Nobody moves UTC forward in spring. That's what makes it a useful anchor: every other time in the world is just UTC with an offset added. To see how UTC relates to the older name GMT, read UTC vs GMT.

An offset says how far a place's clocks are from UTC. A positive offset is ahead, a negative one is behind. To get the local time, add the offset to UTC. At 15:00 UTC on January 15:

• New York (UTC−5): 15:00 − 5 hours = 10:00 on January 15.
• Kathmandu (UTC+5:45): 15:00 + 5 hours 45 minutes = 20:45 on January 15.
• Tokyo (UTC+9): 15:00 + 9 hours = 00:00, which is midnight on January 16. Adding an offset can carry you into the next day, and subtracting one can take you back a day.

You'll often see an offset written at the end of a time, like 2026-01-15T10:00-05:00. That's 10:00 local time in a place five hours behind UTC. To turn it back into UTC, subtract the offset, which here means adding the five hours back: 15:00 UTC.^[2] For more worked examples, see how to convert time zones.

There's no world authority that assigns time zones. Each country decides its own time, usually by law, and can change it whenever it likes.

In the United States, time zones were set by Congress in the Standard Time Act of 1918 and the Uniform Time Act of 1966. The Department of Transportation looks after them today, and either Congress or the Secretary of Transportation can move a boundary. There are nine official US time zones once you count the territories: Atlantic, Eastern, Central, Mountain, Pacific, Alaska, Hawaii–Aleutian, Samoa and Chamorro.^[3]

On a globe, time zones would be neat 15° stripes. In real life, a zone is a group of places that have chosen the same clock rules, and its edges follow country, state and county lines.

Sometimes those choices put a place far from its "natural" zone. Spain sits in line with Britain and Portugal, but in 1940 it moved its clocks forward an hour to match Central Europe, and it has stayed there since. That's one reason Spanish sunsets come so late in the evening.^[4]

A zone's rules can even change inside one state. Most of Arizona stays on standard time all year, but the Navajo Nation, in the state's northeast, follows daylight saving time. The Hopi reservation, which sits inside the Navajo Nation, doesn't.^[5] In summer, driving across that corner of Arizona can mean changing your watch several times.

Governments change their clock rules more often than most people expect. Two recent examples:

• Mexico ended daylight saving time for most of the country in 2022. Clocks went back for the last time on October 30, 2022, but towns along the US border kept the US schedule.^[6]
• Kazakhstan used two time zones until March 1, 2024, when the whole country moved to UTC+5.^[7]

Changes like these are collected in the IANA time zone database, a shared record of the history of local time around the world. It's updated whenever a government changes its time zone boundaries, offsets or daylight saving rules.^[8]

Your devices don't remember New York as "UTC−5". They remember it by a region ID, America/New_York, and look up that region's rules in the time zone database for whatever date they need.^[8] That's how your phone knows New York will be UTC−4 next July, even though it's UTC−5 in January.

Updates to the database reach you through software updates to your phone, computer and browser. A device that hasn't been updated in a long time can show the wrong time for a place that has changed its rules. TimeZonar's world clock reads the same database built into your browser, so it follows each place's latest rules.

In places with daylight saving time, the clock jumps on the change dates. In the United States, the change happens at 2:00 a.m. local time.^[9] That leads to two odd moments each year:

• In spring, clocks jump from 2:00 straight to 3:00. A time like 2:30 a.m. doesn't exist that night.
• In autumn, clocks go from 2:00 back to 1:00. A time like 1:30 a.m. happens twice.

If you're setting an alarm or booking something in the middle of the night on a change date, double-check the time. Learn more in the guide to daylight saving time, and if you write software, see programming with time zones.

1. 1.BIPM, Time metrology. What UTC is, and how leap seconds keep it in step with the Earth's rotation.
2. 2.IETF, RFC 3339: Date and Time on the Internet: Timestamps. How offsets are written in timestamps and how to turn them back into UTC.
3. 3.U.S. Department of Transportation, Uniform Time Act. Who oversees US time zones, how boundaries are changed, and the nine US zones.
4. 4.NPR, Spain's been in the wrong time zone for seven decades. Spain's move to Central European Time in 1940.
5. 5.Associated Press, via NBC San Diego, For Navajo and Hopi tribes, daylight saving is a time of confusion. Daylight saving in Arizona, the Navajo Nation and the Hopi reservation.
6. 6.Bloomberg Línea, Mexico abolishes daylight saving time. Mexico ending daylight saving in 2022, except near the US border.
7. 7.Interfax, Kazakhstan to switch to single time zone. Kazakhstan moving to UTC+5 on March 1, 2024.
8. 8.IANA, Time Zone Database. The shared record of every region's offsets and rule changes.
9. 9.U.S. Department of Transportation, Daylight Saving Time. US clocks changing at 2:00 a.m. local time.