cal_to_jd -- Converts from a supported calendar to Julian Day Count
Description
int cal_to_jd ( int calendar, int month, int day, int year )
cal_to_jd() calculates the Julian day count
for a date in the specified calendar.
Supported calendars are
CAL_GREGORIAN,
CAL_JULIAN,
CAL_JEWISH and
CAL_FRENCH.
