The universal coordinated time (AKA Universel Temps Cordonne - UTC) is a standard time value that is based on an atomic clock, measured and calibrated according to astronomical observations of pulsars.

UTC is synchronized to Greenwich Mean Time which is coincident with the location of the zero meridian.

Local time at any point on the earth's surface is computed relative to UTC by adding a positive or negative offset to UTC. In general, the offsets are measured in complete hours of 60 minutes duration.

However there are some locations in the world where the offset is a fractional offset in hours. As a general rule, each hour corresponds to a movement of 15 degrees further away from the meridian. This is not completely strict since China spans almost 4 hours in terms of distance but is entirely in one time-zone based on the time in Peking.

Some Islamic countries do not adopt the same time scale in any case, choosing instead to measure the time from sunrise to sunset as 12 hours. Since days wax and wane according to the season, this can cause some difficulty in converting local time to UTC and vice versa.