Calculate all time elements from year, DOY and as an option seconds of day
tgps=mjd_t(mjd)
year (I4 or I2. for I2 if yyyy>80 ; yyyy+=1900)
DOY [1-366]
second of day [0-86400]
the function initializes a gpstime structure
tgps =
[tgps]=yyyyddds_t(2009,365,43200.0)