Is the speed of light a constant or is it that we always measure it as a constant since any velocity we have as we measure it would affect the clock we are holding to measure it and thus change the V=d/t equation so that the answer is always the same?

Sorry for being 8 years late for class but I just found out about it. In fact I have already watched half the 11 hr video before finding this site. Sooo looking forward to learning more.