Before the Industrial Revolution, time was kept by local authorities, each town using its own official clock, if it had one.
But with the new-fangled steam engine and railroad enabling people to travel long distances relatively quickly, it became necessary for time to catch up. Hence, the invention of standard time.
Greenwich Mean Time became the first widely used standard in 1884, voted in by the 25 nations at the International Meridian Conference in Washington.
It marks midday as the moment the sun reaches its zenith above the Royal Observatory, in Greenwich, London - the prime meridian already being used to mark longitude in many world maps. Since 1928, Greenwich Mean Time has been called universal time, or UT, and broadcast to the world over radio waves.
Since the atomic clock was invented in the 1950s, more than 400 such devices in laboratories around the world provide another widely used reference called International Atomic Time, or TAI.
This was adopted by the 14th General Conference on Weights and Measures in 1971 as the fundamental basis of the world's time scale, with the atomic definition of the second adopted in 1967 as its unit. But as the earth's rotation is slowing down, this has created a gap between UT and TAI. Since their synchronisation in 1958, TAI is now 34 seconds ahead of UT, according to the International Bureau of Weights and Measures.