What time is it?

Reading the clock

The clock shows the local time of your device’s time zone. Next to it are the zone’s abbreviation and its offset from Coordinated Universal Time (UTC). The offset is the more reliable of the two. EST, for example, always means UTC−5, but New York uses EDT, UTC−4, from March to November. A clock that showed “EST” in July would be wrong by an hour. Offsets also say exactly how far apart two places are: UTC+5:30 in India and UTC−4 in New York are 9 hours 30 minutes apart.

The 24-hour button switches between the two ways of writing the time. On the 24-hour clock the day runs from 00:00 to 23:59. On the 12-hour clock it runs twice from 12:00 to 11:59, first a.m. (ante meridiem, before noon) and then p.m. (post meridiem, after noon). By convention 12:00 a.m. is midnight and 12:00 p.m. is noon, but the US National Institute of Standards and Technology (NIST) calls both ambiguous and advises against them. AM and PM explains the alternatives.

UTC, GMT and Zulu

UTC is the world’s reference time. It is computed by the International Bureau of Weights and Measures from atomic clocks in laboratories around the world, and national laboratories keep real-time versions of it. In the United States, NIST says that UTC(NIST) and UTC(USNO) are kept in agreement, typically to within 20 nanoseconds. The abbreviation was chosen to favour no language: the English initials would be CUT and the French TUC.

Greenwich Mean Time originally meant mean solar time at the Royal Observatory in Greenwich. Its modern astronomical name is UT1, the time kept by the Earth’s rotation. UTC has been held within 0.9 seconds of UT1, so for any clock that shows seconds, GMT and UTC are the same. Two details matter. GMT is also the legal winter time of the United Kingdom, where the law still says “Greenwich mean time” and not UTC. And in summer Britain is on British Summer Time, GMT+1, so “London time” is not GMT for half the year. Pilots, sailors and the military write UTC with a Z, spoken “Zulu”, as in 1430Z.

Daylight saving time

About a third of the world’s time zones move their clocks forward in spring and back in autumn, on dates set by law that differ between regions. So the gap between two cities can change for a few weeks a year: New York is usually 5 hours behind London, but 4 hours for two or three weeks in March and one week around the start of November. When clocks go forward an hour of local time is skipped, and when they go back an hour happens twice. In 2026 British Columbia, Alberta, the Northwest Territories, Manitoba and Morocco stopped changing their clocks, and Cuba cancelled its November change. Daylight saving time has the laws in the US, the EU and elsewhere, and every upcoming change by date.

Where official time comes from

Most computers and phones set themselves with the Network Time Protocol (NTP) from public time servers, or from the mobile network. In the US, NIST’s Internet Time Service runs public NTP servers traceable to UTC(NIST), and its radio station WWVB near Fort Collins, Colorado broadcasts the time on 60 kHz to “atomic” wall clocks and watches. GPS satellites carry time as well, and gps.gov says a receiver can determine the time to within 100 billionths of a second. GPS time does not count leap seconds, so it runs 18 seconds ahead of UTC, and receivers correct for this automatically. NIST’s own website clock, time.gov, corrects for network delay in the same way as this page: it measures the round trip and assumes half of it in each direction.

This page works the same way. It asks our server, which is synchronised to UTC by its operating system, for the time, and corrects for half the round trip. The resulting time is exact to a few hundredths of a second, far better than a display that changes once a second needs. It is meant for setting watches, timing an auction or a ticket release, and checking a device. It is not a traceable time reference for scientific or legal measurement.

Leap seconds

Atomic time and the Earth’s rotation drift apart slowly. To keep UTC within 0.9 seconds of the Earth’s rotation, 27 leap seconds were inserted between 1972 and 2016, each as an extra second 23:59:60 at the end of June or December. The last one came at the end of December 31, 2016. Since then the difference between atomic time (TAI) and UTC has been 37 seconds, and the International Earth Rotation and Reference Systems Service (IERS) has announced no leap second for the end of 2026. In October 2026 the General Conference on Weights and Measures decided that UTC will become continuous, with no further leap seconds, from 20 May 2027. Computer clocks, including this one, follow Unix time, in which every day has exactly 86,400 seconds. On a leap-second night they hold or repeat a second rather than showing 23:59:60.

Setting your device clock right

If the line under the clock says your device is more than a second off, turn on automatic time and let it synchronise:

• Windows 11: Settings › Time & language › Date & time › switch on Set time automatically and Set time zone automatically, then press Sync now.
• macOS: System Settings › General › Date & Time › switch on Set time and date automatically.
• iPhone and iPad: Settings › General › Date & Time › switch on Set Automatically.
• Android: Settings › System › Date & time › switch on Set time automatically (menu names vary by maker).
• Linux: run timedatectl; “System clock synchronized: yes” means NTP is working. sudo timedatectl set-ntp true turns it on.

If the time is wrong by a whole number of hours but the minutes are right, the time zone is wrong, not the clock. Choose the right zone, or let the device set it automatically. Then reload this page: the line under the clock should report an offset of a few hundredths of a second at most.