Julian day

JDs are the recommended way how to specify time in astronomy.

The Julian day or Julian day number (JDN) is the integer number of days that have elapsed since noon Greenwich Mean Time Monday, January 1, 4713 BC. The Julian Date (JD) is the number of days (with decimal fraction of the day) that have elapsed since the same epoch.

The Julian day number can be considered a very simple calendar, where its calendar date is just an integer, and time is its fraction. This is useful for reference, computations, and conversions. The beauty of JD lays in the fact that it is easy to calculate time ranges and to roll dates, since just basic math operations addition and subtraction are used. JD allows the time between any two dates in history to be computed by simple subtraction.

The Julian day system was introduced by astronomers to provide a single system of dates that could be used when working with different calendars and to unify different historical chronologies.