TAI is not distributed directly in every-day life. The time in common use (broadcast by radio, television, the telephone...) is referred to a time scale called Coordinated Universal Time (UTC) as recommended by the 15th CGPM in its Resolution 5 in 1975. UTC differs from TAI by a whole number of seconds. This difference can be modified in steps of 1 s, using a positive or negative leap second, in order to keep UTC in agreement with the time defined by the rotation of the Earth such that, when averaged over a year, the Sun crosses the Greenwich meridian at noon UTC to within 0.9 s. In addition, the legal time of most countries is offset from UTC by a whole number of hours (time zones and "summer time"). National time-service laboratories maintain an approximation of UTC known as UTC(k) for laboratory k. The differences between UTC(k) and UTC are in general no more than a few hundreds of nanoseconds.