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Standard time zones

Earlier, time zones based their time on Greenwich Mean Time (GMT, also called UT1), the mean solar time at longitude 0? (the Prime Meridian). But as a mean solar time, GMT is defined by the rotation of the Earth, which is not constant in rate. So, the rate of atomic clocks was annually changed or steered to

closely match GMT. But on January 1, 1972 it became fixed, using predefined leap seconds instead of rate changes. This new time system is Coordinated Universal Time (UTC). Leap seconds are inserted to keep UTC within 0.9 seconds of UT1. In this way, local times continue to correspond approximately to mean solar time, while the effects of variations in Earth's rotation rate are confined to simple step changes that can be easily subtracted if a uniform time scale (International Atomic Time or TAI) is desired. With the implementation of UTC, nations began to use it in the definition of their time zones instead of GMT. As of 2005, most but not all nations have altered the definition of local time in this way (though many media outlets fail to make a distinction between GMT and UTC). Further change to the basis of time zones may occur if proposals to abandon leap seconds succeed.

Due to daylight saving time, UTC is local time at the Royal Observatory, Greenwich only between 01:00 UTC on the last Sunday in October and 01:00 UTC on the last Sunday in March. For the rest of the year, local time there is UTC+1, known in the United Kingdom as British Summer Time (BST). Similar circumstances apply in many places.

The definition for time zones can be written in short form as UTC?n (or GMT?n), where n is the offset in hours. These examples give the local time at various locations at 12:00 UTC when daylight saving time (or summer time, etc.) is not in effect:

San Francisco, California, United States: UTC?8; 04:00
Saint John, New Brunswick, Canada: UTC?4; 08:00
Toronto, Ontario, Canada: UTC?5; 07:00
Stockholm, Sweden: UTC+1; 13:00
Cape Town, South Africa: UTC+2; 14:00
Mumbai, India: UTC+5:30; 17:30
Seoul, Korea: UTC+9; 21:00
Melbourne, Australia: UTC+10; 22:00
Where the adjustment for time zones results in a time at the other side of midnight from UTC, then the date at the location is one day later or earlier. Some examples when UTC is 23:00 on Monday when daylight saving time is not in effect:

Cairo, Egypt: UTC+2; 01:00 on Tuesday
Wellington, New Zealand: UTC+12; 11:00 on Tuesday
Some examples when UTC is 02:00 on Tuesday when daylight saving time is not in effect:

New York City, New York, United States: UTC?5; 21:00 on Monday
Honolulu, Hawaii, United States: UTC?10; 16:00 on Monday
The time-zone adjustment for a specific location may vary because of Daylight Saving Time. For example New Zealand, which is usually UTC+12, observes a one-hour daylight saving time adjustment during the southern hemisphere summer, resulting in a local time of UTC+13.

See also Sidereal time.