Adjust the system time of your computer using the GPS or NTP, if necessary.
Local
UTC
JD
MJD
GMST
DoY
Caution:
Items: the current system time (Local), coordinated universal time (UTC), julian date (JD), modified julian date (MJD), Greenwich mean sidereal time (GMST), and day-of-year (DoY) with the elapsed time in days from YYYY-01-01 00:00:00 UTC
Formula for computing the JD: http://scienceworld.wolfram.com/astronomy/JulianDate.html