The Julian Date (not to be confused with "Julian calendar" or "Julian year") is a count of the number of days that passed from Jan 1st −4712 (4713 bc) 12:00h, Julian proleptic calendar. For instance, Nov 6, 2013, 03:28:53 h UT becomes 2456602.64506 JD. It is widely use in astronomy to refer to events and epochs.
For possible implementations, check:
http://jodd.org/doc/jdatetime.html
https://github.com/oblac/jodd/blob/master/jodd-core/src/main/java/jodd/datetime/TimeUtil.java#L181