@river: time always has been a measurement of the movement of something. There is a problem with "the movement" of light, though. We can't see light "moving" through space, right? We can, however, measure the frequency of light, so if we use a fixed frequency as a reference we can also fix a distance spatially. There is nothing particularly difficult or weird to understand. What is weird, is that light has no 'time distance', and objects with mass do; light "travels" through space but has no time dimension in spacetime, because photons all lie on the (observer's) lightcone. So any light you see, like from distant stars, has an apparently infinite velocity, or is 'Newtonian' (i.e. instantaneous).