Actually what you want to do is to convert a Gregorian Date to a Year plus day of year.
A Julian Day is really a count of days from January 1 4713 BCE.
The Julian Day Number (JDN) is the integer assigned to a whole solar day in the Julian day count starting from noon Greenwich Mean Time, with Julian day number 0 assigned to the day starting at noon on January 1, 4713 BC proleptic Julian calendar. (November 24, 4714 BC in the proleptic Gregorian calendar.) For example, the Julian day number for 1 January 2000 was 2,451,545.