Originally, 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
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
Tokyo, Japan: UTC+9; 21:00
From Pocket Wikipedia, http://www.free-soft.ro/pocket-wikipedia/