Discover magazine has posted the following Rules for Time Travelers.
Actually, it takes most of the fun out of the whole concept, but in summary they are:
0. There are no paradoxes
1. Traveling into the future is easy
2. Traveling into the past is hard – but maybe not impossible
3. Traveling through time is like traveling through space
4. Things that travel together, age together
5. Black holes are not time machines
6. If something happened, it happened
7. There is no meta-time
8. You can’t travel back to before the time machine was built
9. Unless you go to a parallel universe
10. And even then, your old universe is still there
I strongly encourage the writers of Primeval to read them…
…of course, it would be the end of the show… but that can’t be far off, anyway.