That is the real question... if you pull out your "post-event" stuff and you have not wound your "tin clock" in a while, what do you use for a reference to set it in the first place?

As far as land masses moving... if it comes to that, I don't believe I'll be worrying about setting a clock.

The things I do consider possible are lightening, EMP--for whatever reason, and internet hacks that take down some of our technology... all three can leave you looking at stars for navigation.