Hi, gene heskett wrote: > > In the FWIW dept this time formula is pretty accurate back to the > > middle of 4713 BC.
Greg Wooledge wrote: > Even the *Julian* calendar used in ancient Rome wouldn't have been in > use in 4713 BC. Any calendar would have been locally defined, if one > existed at all. What Gene describes is the Julian Date, an astronomical time counting. I first met it in HP BASIC but it is also used by database systems. 0.000000 is 1st Januar 4713 BC 12:00 UTC (year -4712 because the is no year 0 in BC/AD counting). It must not be confused with the Julian Calendar, which invented the the 4-year leap year rule. This worked until pope Gregor saw the need for a finer adjustment to the day fractions of the astronomical year. This yielded the rules about 100 and 400 years. Afaik, they all suffer from being day-oriented which makes trouble because the astronomical days get longer over time and thus cause leap seconds. For long term accuracy in the range of seconds one needs to work with time countings close to International Atomic Time (TAI) which is nearly perfect but becomes only available up to a month too late. Have a nice day :) Thomas
