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 instead of longitude, because it is convenient for areas in closecommercial or other communication to keep the same time.
Most of the time zones on land are offset from Coordinated Universal Time (UTC) by a whole number of hours (UTC−12:00 to UTC+14:00), but a few zones are offset by 30 or 45 minutes (e.g. Newfoundland Standard Time is UTC−03:30, Nepal Standard Time is UTC+05:45, Indian Standard Time is UTC+05:30 and Myanmar Standard Time is UTC+06:30).
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.
History
Early timekeeping
Before clocks were 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 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).
