Hi All: Third revision.
Thanks to Flavio and Eugene. ## FAQ entry: ######## What time scale does DateTime follow? What's up with UTC, GMT, TAI, and UT1? The DateTime modules are based on a time scale known as UTC (Temps Universel Coordonne, Coordinated Universal Time), which was established internationally in 1972. UTC is widely used in scientific and technical contexts, and has been accepted as the standard time scale for most uses. Civil time generally differs by an integral number of hours from UTC, expressed in time zones. Related time scales include: - GMT (Greenwich Mean Time), in which each twenty-four hour day was exactly 86,400 uniform seconds; GMT was historically based on civil time in London (the "imperial clock"); for time calculations on dates after the introduction of UTC, GMT is equal to UTC; - UT1 (Universal Time), in which day length varies, and is slowly changing, due to irregularities in the earth's rotation detected by astronomical observations (the "earth clock"); - TAI (Temps Atomique International), in which a day has 86,400 seconds whose length is derived from a chosen atomic resonance at sea level (the "atomic clock"). UTC was developed to combine the stable atomic precision of TAI with the earth-sun accuracy of UT1. TAI was originally synchronized with UT1 on 1958 January 1 (i.e., on that date, UT1 - TAI = 0). Because of the slowing down of the rotation of the earth, the TAI - UT1 difference has grown irregularly since 1958. To reconcile the divergence since then between TAI and UT1, UTC is defined to differ from TAI by integer atomic seconds and to differ from UT1 by less than .9 atomic seconds. To maintain this relationship, leap seconds are introduced as needed to the UTC time scale, under the supervision of the International Earth Rotation and Reference Systems Service (<a href="http://www.iers.org/iers";>IERS</a>). As an example, in May 2003, TAI - UT1 was measured to be 32.4 seconds, and TAI - UTC had been incremented to 32 seconds by the IERS. When TAI - UT1 grows to 32.9 seconds, a leap second will be introduced. In the event that TAI - UT1 shrinks -- irregular Earth! -- the introduced leap second would be negative. The DateTime modules account for UTC leap seconds, providing full conformance with UTC. NOTE: As of 2003, there was discussion about UTC's future use of leap seconds. In the event of a change in the UTC definition, the most likely outcome is that the DateTime modules will continue to follow UTC or its successor, and any leap seconds already introduced will be preserved. ## End of FAQ entry ##### - Bruce __bruce__van_allen__santa_cruz__ca__
