Possible input combinations:
(n <- julian_day()) returns the Julian day number for today.
(n = julian_day(y,d)) where y is a single year or a vector of years and d is a single day number or a vector of daynumbers,
returns the date vector [year,month,day] for each year, day pair.
(n = julian_day(y,m,d)) where y is a single year or a vector of years, m is a single month or vector of months, and d is a single month day or a vector of month days,
returns the Julian day number for each year, month, day.