6 months ship time corresponds to ~700 years time on Earth. So how fast is the ship going? Granted the simplifying assp. that the ship's velocity is constant throughout its voyage, it's traveling ~0.999999745C.
Compare: the fastest particles ever produced in accelerators on Earth were at ~0.999999999976C, and the fastest particle ever detected was a cosmic ray proton at ~0.9999999999999999999999951C.