def _clock_to_mjd( clock_seconds ):
    """
    
    Helper function which converts clock time in seconds to MJD in days
    
    """
    # Modified Julian date of the "zero epoch" of the system clock (1/1/70)
    MJD_ZEROPOINT = 40587.0
    # Number of seconds per day.
    SECONDS_PER_DAY = 86400.0
    mjd_days = MJD_ZEROPOINT + (clock_seconds/SECONDS_PER_DAY)
    return mjd_days