Time zones are areas of the Earth that have adopted the same standard time, usually referred to as the local time. Formerly, people used local solar time, originally apparent and then mean solar time. Their difference is the equation of time. Mean solar time is the average over the course of a year of apparent solar time (sundial time). With the expansion of the railways and as telecommunications improved this became increasingly awkward because clocks in a given town would differ from those in any other by an amount corresponding to their difference in geographical longitude, which was generally not a convenient number. Forcing all localities to run their clocks in synchrony would solve this problem, but under such a scheme, in many places the time shown by clocks at sunrise and sunset would differ too markedly from the solar time values to which people are accustomed. As a compromise, a scheme was devised where the surface of the planet was divided into twenty four "time zones", each separated by 15° of longitude and offset by one hour from its neighbor. Under this scheme, local time is always close to mean solar time, while comparing the time in different places is a simple matter of adding or subtracting whole hours. However, the one hour separation is not universal and, as the map below shows, the shapes of time zones can be quite irregular because they usually follow the boundaries of states, countries or other administrative areas.
Originally, time zones were aligned such that the Prime Meridian (longitude 0°) was the centre of its own time zone, with the mean solar time on that meridian (Greenwich Mean Time or GMT) defining its local time. As a result, all other time zones used GMT as the "base time", from which they were offset by a whole number of hours. However, as a mean solar time, GMT, or UT1 as it is also now known, is defined by the rotation of the Earth, which is not constant in rate. Initially, the rate of atomic clocks was annually changed or steered to closely match GMT, but effective 1 January 1972 the rate of atomic clocks became fixed, and time steps (leap seconds) replaced the rate changes. This new time system was called 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 (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.
UTC is, incidentally, local time at Greenwich itself only between 01:00 UTC on the last Sunday in October and 01:00 UTC on the last Sunday in March. For the remainder of the year local time is UTC + 1, known in the UK as British Summer Time (BST). Similar circumstances apply in many places: see Daylight saving time.
The definition for time zones can be written in short form as UTC±n (or GMT±n), where n is the offset in hours.