. There are other synchronization conventions like slow clock transport, just as arbitrary as Einstein's, but having the property that the value of the one-way speed of light according to them is an empirical property of the universe, not just a trivial consequence of our definition.  Clocks and time are not the same thing. Time is a property of nature observable as changes. Day turns to night, summer follows winter, and rivers flow downstream. Clocks, however, are man made objects that do what we tell them to do. For everyday use we regulate clocks to match the noon-to-noon interval,we use stop watches to compare race contestants, and for rocket launches the clocks count backwards. 