//
// tomjd -- Convert date and time to MJD (Modified Julian Day)
//
// 25-Dec-2010 tvb www.LeapSecond.com/tools
//

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

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

void main (int argc, char *argv[])
{
    long mjd, stod, year, month, day, hour, minute, second;
    double fmjd;

    // Get user specified date and time.

    if (argc > 1) {
        year = month = day = hour = minute = second = 0;
        sscanf(argv[1], "%ld-%ld-%ld %ld:%ld:%ld", &year, &month, &day);
        if (argc > 2) {
            sscanf(argv[2], "%ld:%ld:%ld", &hour, &minute, &second);
        }

    } else {
        // By default use current date and time.
        time_t t = time(NULL);
        struct tm *tm = gmtime(&t);
        year   = tm->tm_year + 1900;
        month  = tm->tm_mon + 1;
        day    = tm->tm_mday;
        hour   = tm->tm_hour;
        minute = tm->tm_min;
        second = tm->tm_sec;
        printf("%.24s (current local time)\n", ctime(&t));
    }

    // Convert date to integer MJD (Modified Julian Day).
    // Convert time to integer STOD (Seconds Time Of Day).
    // Combine to form floating point MJD.

    mjd = date_to_mjd(year, month, day);
    stod = (((hour * 60) + minute) * 60) + second;
    fmjd = mjd + (stod / 86400.0);

    printf("%.4ld-%.2ld-%.2ld %.2ld:%.2ld:%.2ld UTC = MJD %ld + STOD %ld = MJD %.6lf\n",
           year, month, day, hour, minute, second, mjd, stod, fmjd);
}

//
// Return Modified Julian Day (MJD) given
// calendar year, month (1-12), and day (1-31).
// - Valid for Gregorian dates from 17-Nov-1858.
// - Adapted from sci.astro FAQ.
//

long date_to_mjd (long year, long month, long day)
{
    return
        367 * year
        - 7 * (year + (month + 9) / 12) / 4
        - 3 * ((year + (month - 9) / 7) / 100 + 1) / 4
        + 275 * month / 9
        + day
        + 1721028
        - 2400000;
}

#if 0

>tomjd
2010-12-24 22:13:41 UTC = MJD 55554 + STOD 80021 = MJD 55554.926169

>tomjd 2000-1-1
2000-01-01 00:00:00 UTC = MJD 51544 + STOD 0 = MJD 51544.000000

>tomjd 2010-12-25
2010-12-25 00:00:00 UTC = MJD 55555 + STOD 0 = MJD 55555.000000

>tomjd 2010-12-25 13:20
2010-12-25 13:20:00 UTC = MJD 55555 + STOD 48000 = MJD 55555.555556

#endif