source: buchla-68k/orig/DATE/DATETX.C@ 6888aa2

/*
 *              Convert Date to Readable Format.
 *
 * Synopsis:
 *
 *      char    *datetxt(buffer, year, month, day);
 *      char    *buffer;        -- Output string goes here
 *      int     year;           -- Year,        1979 = 1979
 *      int     month;          -- Month,       January = 1
 *      int     day;            -- Day,         1 = 1
 *
 * The readable date will be written into the outpub buffer, terminated by
 * a null byte.  datetxt returns a pointer to the null byte.
 *
 * External routines called:
 *
 *      nbrtxt          (Number to ascii conversion)
 *      copyst          (String copy routine)
 */

extern  char    *nbrtxt();
extern  char    *copyst();
extern  char    *datetxt();

static char *daynames[] = {
        "Sunday",                       /* Sunday is day zero           */
        "Monday",
        "Tuesday",
        "Wednesday",
        "Thursday",
        "Friday",
        "Saturday",
};

static char *monthnames[] = {
        "?Nomember?",                   /* Illegal month                */
        "January",
        "February",
        "March",
        "April",
        "May",
        "June",
        "July",
        "August",
        "September",
        "October",
        "November",
        "December",
};

char *datetxt(buffer, year, month, day)
char    *buffer;                        /* Output goes here             */
int     year;                           /* Year, 1979 = 1979            */
int     month;                          /* Month of year, Jan = 1       */
int     day;                            /* Day in the month 1 = 1       */
/*
 * Output the date in readable format:
 *      Tuesday, the third of October
 */
{
        register char   *op;                    /* Output pointer       */

        op = buffer;                            /* Setup output pointer */
        op = copyst(op, daynames[dayofweek(year, month, day)]);
        op = copyst(op, ", the ");
        op = nbrtxt(op, day, 1);
        op = copyst(op, " day of ");
        op = copyst(op, monthnames[(month < 0 || month > 12) ? 0 : month]);
        op = copyst(op, ", ");
        if (year < 1000 || year >= 2000)
                return(nbrtxt(op, year, 0));
        else {
                op = nbrtxt(op, year/100, 0);
                op = copyst(op, " ");
                if ((year = year % 100) == 0)
                        return(copyst(op, "hundred"));
                else
                        return(nbrtxt(op, year, 0));
        }
}

dayofweek(year, month, day)
int     year;                                   /* Year, 1978 = 1978    */
int     month;                                  /* Month, January = 1   */
int     day;                                    /* Day of month, 1 = 1  */
/*
 * Return the day of the week on which this date falls: Sunday = 0.
 * Note, this routine is valid only for the Gregorian calender.
 */
{
        register int yearfactor;

        yearfactor = year + (month - 14)/12;
        return (( (13 * (month + 10 - (month + 10)/13*12) - 1)/5
                + day + 77 + 5 * (yearfactor % 100)/4
                + yearfactor / 400
                - yearfactor / 100 * 2) % 7);
}