I recognize the analytical usefulness of having time, a parameter. However, does time really exist? If so, is there a universal time? Can we measure time without a clock, since clocks are just spatial units we can compare with. I notice how in all texts we always have another observer's clock shifted....but what if the clock shifted due to length contraction etc. How can there be a definite measurement of time in a certain frame when there is no way to check its accuracy?