source: buchla-68k/orig/DATE/JDAY.C

/*
   =============================================================================
        jday.c --  convert date to Julian day. -- ACM algorithm 199

        A Julian day is the number of days since some base date
        in the very distant past.

        Handy for getting date of x number of days after a given Julian date
        Use jdate() to get that from the Gregorian date.

        Written by:             Robert G. Tantzen

        Translated to C by:     Nat Howard

        Revised by:             D.N. Lynx Crowe
                1998-01-14 -- optimized for speed

        Translated from the algol original in Collected Algorithms of the <ACM
        This and jdate() are algorithm 199.
   =============================================================================
*/

long
jday(mon, day, year)
int mon, day, year;
{
        register long y = year;
        register long m = mon;
        register long j;
        register long ya;
        register long c;
        long d = day;

        if(m > 2) {

                m -= 3L;

        } else {

                m += 9L;
                --y;
        }

        c = y / 100L;
        ya = y - (100L * c);

        j = (146097L * c) / 4L
                + (1461L * ya) / 4L
                + (153L * m + 2L) / 5L
                + d + 1721119L;

        return(j);
}