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Astromomers often measure time using the Julian Date and the
Modified Julian Date. The Julian date is the number of days
since the "begining of time" as estimated by some one from the
god squad of the Middle Ages. The day begins at noon GMT, and
today's date at 1PM EST is JD2446829.3. The Julian date is
useful because there is no need to worry about things like
leap years and changes in the calendar when measuring time
differences. Another useful unit of measurement is the
Modified Julian Date. It is the number of years after
0, January 1, noon GMT. The number looks like the year we
are used to. Today's date is J1987.2. Useful conversions:
J2000.0 = 2000 January 1, noon GMT = JD2451545.0.
J2000.0 is the origin of many formulas that give the position of
the sun and the moon, and these formulas often measure time in
units of days. Hence, my suggestion for (days-after-J2000.0).