If you see a spacecraft moving past you at 90% of the speed of light, and if you are able to watch a clock on the spacecraft, then according to special relativity you will see the clock

Measures a slower course of time

Explanation:

According to the theory of relativity, there is a difference in the time measured by two observers, due to a relative speed difference between them. In other words, you observe that a clock that moves with respect to you will measure a slower course of time than a clock that is at rest in your frame of reference.