What is Time?

Time is the way we measure change and sequence events — from the ticking of atoms in laboratories to the daily rising of the Sun. This page explains how modern timekeeping works, why hours, minutes and seconds exist, and what keeps our clocks honest.

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Indian Standard Time • IST (UTC +5:30)

UTC

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Sidereal day

23:56:04

Solar day

24:00:00

Since last leap sec

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Atomic clock

Cesium standard • 9,192,631,770 Hz

Local analog clock

Synchronized to system time. Uses civilian civil time (IST).

Time since the Big Bang

— years • — days • —:—:—

How seconds were defined

Atomic transition • 9,192,631,770 Hz

Since 1967 the second is defined by cesium-133 atomic transitions.

Why 24 hours?

Historical sexagesimal system

Ancient Egyptians and Babylonians split days; base-60 arithmetic made fractions easy.

UTC vs Civil Time

UTC · IST (+5:30)

Coordinated Universal Time is the modern reference; local civil time adds offsets (e.g. IST +5:30).

Origins of timekeeping

Early humans used the Sun and Moon to mark days and months. Sundials and water clocks (clepsydra) tracked daytime and night-time hours. Mechanical escapement clocks in medieval Europe allowed continuous counting of hours — and led to the daily schedules of modern life.

~3500 BCE: Egyptian sundials and decans used to divide night into segments.

~500 BCE: Babylonian base-60 fractions popularized; helped create 60 minutes and 60 seconds subdivisions.

17th century: The pendulum clock improved accuracy and enabled scientific experiments in time measurement.

1955–1967: Atomic clocks matured; in 1967 the SI second was redefined using cesium transitions.

Why we still correct time (leap seconds)

The Earth's rotation slows slightly due to tidal friction, so astronomers sometimes insert leap seconds into UTC to keep civil time aligned with the mean solar day. Leap seconds are announced by the International Earth Rotation and Reference Systems Service (IERS). They are rare and unpredictable.

Last leap second added: 31 December 2016 (UTC). Because leap seconds are scheduled irregularly, there is no permanent countdown until IERS announces one.

How we handle it here

If a leap second is announced, clocks are adjusted on the announced UTC timestamp.
Atomic clocks continue to measure SI seconds reliably — GPS and international time services manage the correction.