If I (on the Earth) and you (on a very fast rocket) measure the time that takes you to travel a certain distance we would not agree. Still, nothing forbids me to compute your velocity using my measures of time and distance. The interesting point is that if your rocket is made of photons, even a third person trying to chasing you will agree with my measured speed. This happens because of the combination of time dilation and length contraction (the person moving with respect to me see a shorter distance, but his time also goes slower, both of the same factor $\gamma$), he will always agree with me unless he reaches your photonic speed somehow braking relativity. That is just how relativity works: time is not absolute, speed of light is!

If I (on the Earth) and you (on a very fast rocket) measure the time that takes you to travel a certain distance we would not agree. Still, nothing forbids me to compute your velocity using my measures of time and distance. The interesting point is that if your rocket is made of photons, even a third person trying to chasing you will agree with my measured speed. This happens because of the combination of time dilation and length contraction (the person moving with respect to me sees a shorter distance, but his time also goes slower, both of the same factor $\gamma$), he will always agree with me unless he reaches your photonic speed somehow braking relativity. That is just how relativity works: time is not absolute, speed of light is!