The time-scale that is provided by broadcast time signals, and is the basis for all civil timekeeping. The natural choice for this standard time would be Universal Time (UT), but that contains unpredictable irregularities caused by changes in the rotation rate of the Earth. Consequently UTC is linked to International Atomic Time (TAI), but an offset is introduced to keep it always within 0.9 s of UT. This is done by introducing an occasional leap second. The time-scale now widely known as GMT is in fact UTC.