With the time dilatation (and probably some other factors) one cannot simply rely on ones watch.
Ultimately, a "perfect watch" is all one can rely upon. A good approximation of a "perfect watch" is an atomic clock that is located at the center of mass of your (relativistic) spacecraft.
The basic problem is that simultaneity is relative even in special relativity. In general relativity, the simultaneity of relativity is a manyfold aggravated issue. Ben Franklin's statement that "Three can keep a secret, if two of them are dead" is very applicable to relativistic mechanics. How do you keep time, synchronously? As long as more than one are trying to do it, you can't.
We can (kind of) pretend that relativistic effects are linear, and in that case you end up with something like JPL's ephemeris time Teph or the IERS's/IAU's TCB (barycentric coordinate time) or TDB (barycentric dynamic time). None of them are perfect; they can't be.