Timezone

A time zone is a region of the globe that observes a uniform standard time for legal, commercial, and social purposes. Time zones tend to follow the boundaries of countries and their subdivisions because it is convenient for areas in close commercial or other communication to keep the same time. A display of standard time may be based on either mean solar time, as shown by clocks and watches generally, or Coordinated Universal Time, which is displayed by radio-controlled clocks. The difference between the two timescales is not permitted to exceed 0.9 second - where necessary a leap second will be inserted into coordinated universal time to ensure this limit is maintained.

National laws specify whether the legal time for general purposes is based on Greenwich Mean Time (which is mean solar time at zero longitude) or coordinated universal time, and the zone offset (if any) to be applied. The legal time is paramount. Both GMT and UTC may be referred to as "Zulu time". Most of the time zones on land are offset from Zulu time by a whole number of hours (−12 to +14), but a few zones are offset by 30 or 45 minutes (for example Newfoundland Standard Time is GMT−03:30, Nepal Standard Time is GMT+05:45, and Indian Standard Time is GMT+05:30). The European Union employs Greenwich Mean Time in its directives. Some higher latitude and temperate zone countries use daylight saving time for part of the year, typically by adjusting local clock time by an hour. Many land time zones are skewed toward the west of the corresponding nautical time zones. This also creates a permanent daylight saving time effect.

Before clocks were first invented, it was common practice to mark the time of day with apparent solar time (also called "true" solar time) – for example, the time on a sundial – which was typically different for every location and dependent on longitude.

When well-regulated mechanical clocks became widespread in the early 19th century, each city began to use some local mean solar time. Apparent and mean solar time can differ by up to around 15 minutes (as described by the equation of time) because of the elliptical shape of the Earth's orbit around the Sun (eccentricity) and the tilt of the Earth's axis (obliquity). Mean solar time has days of equal length, and the difference between the two sums to zero after a year.

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