Nope. You notion on the speed of light carries an assumption; one that is illustrated by the fact that light can be slowed (it’s been done), it can be imaged (cool paradox, eh?), and even imaged such to show it’s duality, and it can be frozen (this has been done as well). And yes, we can tell when it has. The speed of light does not change time, that would be a misunderstanding of Special Relativity. In the simple equation e = mc2 what is C? Though many refer to it as the speed of light, it isn’t, yet it seems conventional to use that euphemism...why? Answering that question will go a ways in clearing confusion as well as why the euphemism is still used, though it is misleading.