Time is whatever a clock counts. People spent several thousand years learning to count it well.
--:--:--
Your device · UTC --:--:--
The short answer
Ask physicists what time is and you get three answers. Each is correct in its own territory.
Newton, 1687Time as a riverAbsolute, and the same for everyone. It flows at one rate whatever happens. It is also how most of us still feel it.
Einstein, 1905 and 1915Time as a dimensionA clock runs slower when it moves fast or sits deeper in gravity. GPS satellites gain about 38 microseconds a day from this, and the system corrects for it.
ThermodynamicsTime as a directionEggs break and do not unbreak. The one-way growth of disorder, called entropy, is the best account we have of why the past differs from the future.
None of these says what time is underneath. Philosophers have argued about that for as long as there have been philosophers. Around the year 400, Augustine described the problem well: he understood time perfectly until someone asked him to explain it.
In practice, time is a number a clock produces. The rest of this page is about how that number got so good.
Why 60 and 24
Our units are fossils from older systems. The Babylonians counted in sixties, a number that divides evenly by 2, 3, 4, 5, 6, 10, 12, 15, 20 and 30. That gave us 60 minutes, 60 seconds and 360 degrees in a circle.
The Egyptians told the daytime hours from shadows and the night hours from rising stars, and used twelve of each, which is where 24 comes from. Those early hours were not equal. A summer daylight hour was longer than a winter one. Fixed-length hours came from Greek astronomers and spread slowly, then quickly once mechanical clocks put them on a tower for everyone to see.
The seven-day week matches no astronomical cycle exactly. It came from Babylonian and Jewish practice, picked up planetary names in the Roman world, and spread with Christianity and Islam.
The history, in order
c. 2000 BCE
Counting in sixtyMesopotamian scribes use base 60, the source of the 60-minute hour and the 360-degree circle.
c. 1400 BCE
Shadows and dripping waterEgypt has sundials and water clocks, and splits day and night into twelve hours each. The hours stretch and shrink with the seasons.
2nd century BCE
Equal hoursGreek astronomers, Hipparchus among them, argue for hours of fixed length. Everyday life keeps using seasonal hours for centuries.
45 BCE
The Julian calendarRome fixes the year at 365.25 days, with a leap day every fourth year.
1088
A clock tower in KaifengSu Song builds a water-driven astronomical clock in China with an escapement, a device that releases power in even steps.
Late 1200s
Clocks that ringWeight-driven mechanical clocks appear in European towns and monasteries. Many have no face and simply strike a bell.
1582
The Gregorian calendarPope Gregory XIII drops ten days and trims the leap-year rule so the calendar stays level with the seasons. Britain waits until 1752.
1656
The pendulumChristiaan Huygens builds the first pendulum clock. Errors fall from minutes a day to seconds a day.
1761
A clock for the seaJohn Harrison's H4 chronometer loses only seconds on a long voyage. Knowing the time at home port lets a navigator work out longitude.
1840 to 1883
Railway timeTrains turn every town's local noon into a problem. Britain's Great Western Railway adopts London time in 1840, and North American railroads adopt standard time zones on 18 November 1883.
1884
GreenwichAn international conference in Washington chooses the Greenwich meridian as the zero line for longitude and for the world's day.
1905 and 1915
Time stops being absoluteEinstein shows that elapsed time depends on speed and on gravity.
1927
QuartzWarren Marrison at Bell Labs builds the first quartz clock, steadier than any pendulum.
1955
The atomic clockLouis Essen and Jack Parry run the first practical caesium atomic clock at the National Physical Laboratory in England.
1967
The second changes ownerThe second is redefined by caesium atoms instead of the rotation of the Earth.
1971
Clocks on planesJoseph Hafele and Richard Keating fly atomic clocks around the world on airliners. The clocks return slightly out of step with one left on the ground, as relativity predicts.
1972
UTCCoordinated Universal Time begins, with leap seconds to keep it close to the Earth's rotation.
2022
The end of leap secondsThe world's measurement authorities agree to stop adding leap seconds by 2035.
The second today
9,192,631,770
Cycles of caesium-133 radiation make one second
Since 1967 the second has been defined by an atom instead of the sky. It is that many cycles of the microwave radiation a caesium-133 atom gives off as it switches between two energy states.
Atomic clocks keep steadier time than the spinning Earth, which wobbles and is slowly braking. Civil time, called UTC, follows atomic time but is held within a second of the Earth's rotation by adding leap seconds. Twenty-seven have been added since 1972, the last on 31 December 2016.
The best laboratory clocks now count the much faster cycles of visible light, and would drift by about a second over many billions of years. The clock on your phone is a distant descendant of them, corrected over the network.