//
// Simple tool to convert fractional MJD to UTC and local time.
// Valid for modern MJD (1970 to 2038).
//

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

char *zonename (char *name)
{
    int i;
    char *p;
    static char zone[4];

    i = 0;
    for (p = name; *p && i < 3; p += 1) {
        if (*p >= 'A' && *p <= 'Z') {
            zone[i] = *p;
            i += 1;
        }
    }
    zone[i] = '\0';

    return zone;
}

#define MJD_1970_01_01 40587

#define UNIX_TIME_TO_MJD(t) ( MJD_1970_01_01 + ((t) / 86400.0) )
#define MJD_TO_UNIX_TIME(m) ( (long) (((m) - MJD_1970_01_01) * 86400.0) )

#define SAFE_CTIME(t) ( ctime(t) ? ctime(t) : "null" )
#define SAFE_TM(tm) ( (tm) ? (tm)->tm_isdst : 0 )

void show (time_t Time, double Mjd)
{
    struct tm *tm;
    int zone;
    int dst;

    if (Time == 0) Time = MJD_TO_UNIX_TIME(Mjd);
    if (Mjd == 0.0) Mjd = UNIX_TIME_TO_MJD(Time);

    printf("MJD %.6lf =\n", Mjd);

    zone = _timezone;
    dst = _daylight;
    {
        _timezone = 0;
        _daylight = 0;
        printf("    %.24s %s\n", SAFE_CTIME(&Time), "UTC");
    }
    _timezone = zone;
    _daylight = dst;

    tm = localtime(&Time);
    printf("    %.24s %s\n", SAFE_CTIME(&Time), zonename(_tzname[SAFE_TM(tm)]));
}

void main (int argc, char *argv[])
{
    double Mjd;
    time_t Time;

    if (argc == 1) {
        time(&Time);
        show(Time, 0);

    } else while (argc > 1) {
        sscanf(argv[1], "%lf", &Mjd);
        show(0, Mjd);
        argc -= 1;
        argv += 1;
    }
}