Here we perform a thought experiment.

Defined are two systems, 1st the "observer"-O, with a clock at 0,0,0 stationary. 2nd the "blinker"-B who is a clock, blinking a light on once per second.

At t=0 B and O are at 0,0,0

Let the blinker instantly accelerate to a velocity of 100m/s in the X axis.

After 100 seconds where is B?
What is B's apparent velocity as measured from O?
What is the apparent period between B's blinks as measured from O?
What if any time dilation has B "experienced?"

After 100 seconds let B's velocity invert to -100m/s
At t=200 seconds where is B and how many blinks did it do?
How many blinks did O observe?