Systems of Time

"Now let me see," the Golux said. "If you can touch the clocks and never start them, then you can start the clocks and never touch them. That's logic, as I know and use it...."
-- James Thurber, "The 13 Clocks"

Delta T is the difference between Earth rotational time (UT1) and dynamical time (TDT). Predicted values of UT1 - UTC are provided by the Earth Orientation Department. An example showing the variation of the length of the day to late 2008 is shown below. Units are milliseconds.

Julian Day Number is a count of days elapsed since Greenwich mean noon on 1 January 4713 B.C., Julian proleptic calendar. The Julian Date is the Julian day number followed by the fraction of the day elapsed since the preceding noon.

We frequently make use of the Modified Julian Date (MJD), which is defined as MJD = JD - 2400000.5. An MJD day thus begins and ends at midnight. Julian dates can be expressed in UT, TAI, TDT, etc. and so for precise applications the timescale should be specified, e.g. MJD 49135.3824 TAI.

To see the current MJD, press this button: .

For more information, see:
Explanatory Supplement to the Astronomical Almanac, P. K. Seidelmann, ed., University Science Books, 1992, ISBN 0-935702-68-7