JULIAN DATE
The Julian Date (JD), or Julian Day Number, is a running
count of days that starts at noon on January 1, 4713 BCE of the
Julian Calendar. The date changes at noon, Greenwich (Universal)
Time. It is widely used for astronomical observations,
particularly for variable stars, so as to avoid calendrical
confusion and to allow for easy subtraction of dates. The US Naval
Observatory has a fuller explanation and a
converter. The American Association of Variable Star
Observers maintains annual Julian
Date calendars.