Of course, once you start dealing with astronomical distances and speeds, the whole concept of a single universal measure of time kind of goes out the window anyway so it's probably going to get worse, not better. :P

(FWIW my definition of 'best' here is something along the lines of 'simple, no special rules or tinkering, gets translated to a human-readable local time on demand'. Which is pretty much how Unix epoch works, come to think of it.)