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

/*
   =============================================================================
        weekof.c -- return the Monday of the week a given Julian day is in
        Version 1 -- 1988-01-20 -- D.N. Lynx Crowe

        Accepts and returns Julian day numbers as used with jdate() or jday().
   =============================================================================
*/

#include "stddefs.h"

long
weekof(jd)
long jd;
{
        short  wd;

        wd = (jd + 1L) % 7;

        if (wd EQ 1)
                return(jd);
        else if (wd)
                return(jd - (long)(wd - 1));
        else
                return(jd - 6L);
}