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In physics, time is defined as the distance between events. Special relativity showed that time cannot be understood except as part of spacetime, a combination of space and time. The distance between events now depends on the relative speed of the observers of the events. General relativity further changed the notion of time by introducing the idea of curved spacetime. Time can be measured, just like other physical dimensions. Measuring devices for time are clocks. Very accurate clocks are often called chronometers. The best available clocks are atomic clocks. The standard unit for time is the SI second, from which larger units are defined like the minute, hour, day, week, month, year, decade, and century.

There are several continuous time scales in current use: atomic time, universal time, ephemeris time. Mankind has invented all sorts of calendars to track the passages of days, weeks, months, and years in various ways.

See also: Synchronization, ISO_8601.