JD = (1461 x (Y + 4800 + (M – 14)/12))/4 + (367 x (M -2 -12 x ((M – 14)/12)))/12 – (3 x ((Y + 4900 + (M – 14)/12)/100))/4 + D – 32075
where JD=julian date number, Y=year, M=month, D=day of month
and valid for JD≥0, date after -4713 November 23.
source:
Cox A.N., “Allen’s Astrophysical Quantities”, New York, Springer-Verlag, 2000 (4rth ed.)