Well both special and general relativity require relative time.
I suppose Bob might like Einstein's paper on the photoelectric effect or be impressed by the awesome predictive power of his theories of relativity.
I doubt it's the latter though, because as I recall it, Bob compared the effectiveness of relativity to the failed epicycle theory which is, of course, a spurious comparison. After all, epicycles were developed to shoehorn existing observations into the assumed world view of the day, whereas GR was entirely theoretical in its inception, although it also built on special relativity which reconciled various experimental observations of the past that were not in accordance with Newtonian mechanics and also provided a theoretical framework by which physicists could dispense with the idea of luminiferous aether, an idea that was not supported by observation.
There have been a number of experimental tests of aspects of GR (not all of them involving clocks) and so far it has passed all of them. This is very significant because GR is a "sudden death" theory - if it fails any of the tests then it fails completely.
Even if it continues to be demonstrated in future experiments, we already know that GR is not completely correct because, like Newtonian mechanics, it doesn't tell the whole story: for example, it doesn't account for quantum gravity. However unless it is disproven by experimentation, any subsequent explanation will have to encompass GR in the same way that GR encompasses Newtonian mechanics.
