Julian Dates Time is one of the most important quantities in any physical system. Hence we need an efficient method for recording time. The usual system of calendar dates is clumsy, for several reasons. For one thing, the calendar system has changed many times in the past. There is no year zero, even though any numbering system should include 0.
Astronomers simplify their timekeeping by simply counting the days.
The Julian Day begins at noon, Greenwich Mean Time. Clock time after noon is converted to a fraction of a day. This fraction of a day beyond noon is added to the whole-number JD to compute the JD of any event or observation.