Calculate all time elements from y,m,d,hh,mm,ss
tgps=mjd_t(mjd)
year (2 or 4 digits), month (1-12), day (1-31)
hour, minute (, second (0-59.999)
the function initializes a gpstime structure
tgps =
[tgps]=ymdhms_t(2009,12,31,12,0,0)