//
// Convert Gregorian date to Modified Julian Date (MJD)
//
// MJD     0 is 1858-11-17
// MJD 51544 is 2000-01-01
// MJD 99999 is 2132-08-31
//
// 26-Nov-2005 /tvb
//

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

long calc_mjd (long year, long month, long day);

void main (int argc, char *argv[])
{
    long mjd;
    time_t t;
    struct tm *tm;
    int year, month, day;

    time(&t);
    tm = gmtime(&t);

    year   = (argc > 1) ? atoi(argv[1]) : 1900 + tm->tm_year;
    month  = (argc > 2) ? atoi(argv[2]) : 1 + tm->tm_mon;
    day    = (argc > 3) ? atoi(argv[3]) : tm->tm_mday;

    mjd = calc_mjd(year, month, day);

    printf("%.4d-%.2d-%.2d = MJD %ld\n", year, month, day, mjd);
}

// Convert Gregorian date to Modified Julian Date (MJD)

long calc_mjd (long year, long month, long day)
{
    if (month < 3) {
        if (month == 2) {
            day += 1;
        }
        month += 12;
        year -= 1;
    }
    day += (year * 365) + (year / 4) - (year / 100) + (year / 400);
    day += (489 * month) / 16;
    return day - 678973;
}