g2jd converts a calendar date to its julian day.
The julian day is the relative day number since January 1, 4713 B.C. The day starts at noon. The fraction part of the day represents the time.
The g2jd is used to determine the julian day range in the daterange field of datagen.
The syntax for g2jd is:
g2jd year month day hour
g2jd returns the julian day