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

/*
   =============================================================================
        jdate.c -- Julian date converter -- ACM Algorithm 199

        Algorithm 199 from Collected algorithms of the ACM.

        Takes a julian date (the number of days since some distant epoch
        or other), and a pointer to a result array.  Returns a pointer to the
        filled in result array:

                ip[0] = month;

                ip[1] = day of month;

                ip[2] = year (actual year, like 1977,
                        not 77 unless it was  77 a.d.);

                ip[3] = day of week (0->Sunday to 6->Saturday)

        The Gregorian calendar is assumed.

        Written by:             Robert G. Tantzen

        Translated to C by:     Nat Howard

        Revisions by:

                D.N. Lynx Crowe         1988-01-14
                        optimized for speed and adapted for re-entrancy
   =============================================================================
*/

/* 

*/

int *
jdate(j, ret)
register long j;
register int *ret;
{
        register long d, m, y, k;

        ret[3] = (j + 1L) % 7L;

        j -= 1721119L;

        k = (4L * j) - 1L;
        y = k / 146097L;
        j = k - (146097L * y);
        d = j / 4L;

        k = (4L * d) + 3L;
        j = k / 1461L;
        d = k - (1461L * j);
        d = (d + 4L) / 4L;

        k = (5L * d) - 3L;
        m = k / 153L;
        d = k - (153L * m);
        d = (d + 5L) / 5L;
        y = (100L * y) + j;

        if(m < 10)  {

                m += 3;

        } else {

                m -= 9;
                ++y;
        }

        ret[0] = m;
        ret[1] = d;
        ret[2] = y;

        return(ret);
}