Universal Coordinated Time

The extreme accuracy of the atomic clocks made soon desirable to adopt the atomic time as the official measure of time in the world, in substitution of the universal time, very less accurate. But the great accuracy of the atomic clocks makes the atomic time not directly useful for every day life, ruled by solar time. Indeed, the atomic clocks made more evident what was known before with less accuracy by astronomical observations: the continuous slowdown of the Earth rotation.

This phenomenon, mainly due to the braking action of the tides, increases the duration of the mean solar year of about 0.8 second a century, about 1 hour every 450.000 years. Since 1900, the year when 1 mean solar second = 1 atomic second, until today, the mean solar day is grown 0.002 atomic second longer and consequently the universal time has a delay of about 1 second every 500 days in comparison with the international atomic time. To take into account this progressive phase displacement between the two time scales and to not give up the high accuracy offered by atomic clocks, in 1972 it was found a compromise that led to the definition of the Universal Coordinated Time (UTC), currently used as the official time measurement in the world. The UTC time flows essentially like the international atomic time (TAI), when the difference with the universal time (UT) approach 1 second, it is inserted 1 second, called leap second, into the UTC time scale in such a way to keep this difference below 0.9 second. The insertion of the leap second is stated by the International Earth Rotation Service (IERS) that continuously monitors the Earth rotation speed. The preferred dates for the leap second insertion are June 30 and December 31.

Other information

The historical series of the leap second
Records since 1972 all dates of the leap second insertion and the progressive delay between UT time and TAI time.

The leap second bulletin (IERS bulletin C)
To known immediately and directly from IERS when will be inserted the next leap second, everybody can subscribe to this bulletin that is delivered by electronic or surface mail.

F. Pollastri - The Time of Internet