If by "Julian date" you mean a date on the Julian calendar, then the GregorianCalendar API tells you how, under the method setGregorianChange(Date):
To obtain a pure Julian calendar, set the change date to Date(Long.MAX_VALUE). To obtain a pure Gregorian calendar, set the change date to Date(Long.MIN_VALUE).
Now I have no idea why anyone would want to use the Julian calendar nowadays, but that is the correct meaning of "Julian date" - a date on the Julian calendar. However some people use the term as lazy (and incorrect) shorthand for "Julian day number", which is the number of days elapsed since noon on 1 January 4713 B.C. (GMT). (Useful for astronomy.) In that case, the link Jamie Robertson gave should be all you need. Or you can perform the calcs yourself by creating a Date for Greenwich mean noon on 1 January 4713 BC, and then compare the getTime() value (in milliseconds) with any other Date you may encounter. You may even use Thomas Paul's new TimeSpan class if you like, once he finishes debugging it. [ January 29, 2003: Message edited by: Jim Yingst ]
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subject: [help] any code to convert YYYYMMDD to Julian Date?