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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.