Hi All: Here's a second revised FAQ covering DateTime's use of the UTC time scale. If this is OK (Dave and Flavio?), Ben, could you add it to the DT FAQ?
Some may have seen the news that debate has flared up regarding the continued use of leap seconds. I don't know whether it will be resolved soon, so it seemed best to simply acknowledge the issue, and hope we remember to update the FAQ if something radically changes. This FAQ entry gets longer every revision, but it seems important for acceptance of DT that we have a fairly rigorous explanation. ## 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 (Coordinated Universal Time), which was established internationally in 1972. UTC is widely used in scientific and technical contexts, and is increasingly accepted as the standard time scale for civic and business uses. Related time scales include: - GMT (Greenwich Mean Time), in which each twenty-four hour day has exactly 86,400 uniform seconds by international convention (the "imperial clock"); - 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). 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>). 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, and any leap seconds already introduced will be preserved. ## End of FAQ entry ##### - Bruce __bruce__van_allen__santa_cruz__ca__
