First idea I have when we talk about calendars in Java: take a look at the JodaTime library.

when you look at wikipedia
you find some source code in the external links
e.g. (I don't know if it's exactly right for you but there are others)

see also:

http://en.wikipedia.org/wiki/Julian_day#Converting_Julian_or_Gregorian_calendar_date_to_Julian_Day_Number


(it's basic  )

1. 800 REM   CALENDAR --> JD
2. 805 REM
3. 810 INPUT "Y,M,D ";Y,M,D
4. 815 INPUT "JC (0) OR GC (1) ";G
5. 820 D1=INT(D): F=D-D1-0.5
6. 825 J=-INT(7*(INT((M+9)/12)+Y)/4)
7. 830 IF G=0 THEN 850
8. 835 S=SGN(M-9): A=ABS(M-9)
9. 840 J1=INT(Y+S*INT(A/7))
10. 845 J1=-INT((INT(J1/100)+1)*3/4)
11. 850 J=J+INT(275*M/9)+D1+G*J1
12. 855 J=J+1721027+2*G+367*Y
13. 860 IF F>=0 THEN 870
14. 865 F=F+1: J=J-1
15. 870 PRINT "J.D.: ";J;F
16. 875 END
17. 880 REM  ------------------------
18. 885 REM  APPEARED IN ASTRONOMICAL
19. 890 REM  COMPUTING, SKY & TELE-
20. 895 REM  SCOPE, MAY, 1984
21. 900 REM  ------------------------
22. 

Greetings, Chrisir

i will also try to port that to processing, thanks a lot to you too chrisir :) will post results here ( wiki .. maybe i wasn't as thorough as i should've been with my search)

greetings,
squallevan