This function converts a date in the Gregorian calendar to a Julian day number (i.e., a method of consecutive numbering of days---does not have anything to do with the Julian calendar!)
julian_day(y, m, i)
double, day of month.
double, Julian day.
valid for dates after -4712 January 1 (i.e., jde >= 0)
Meeus, J. 1991. Chapter 7 "Julian Day". Astronomical Algorithms. Willmann-Bell.