Thanks for the info evildictaitor, I'll just have to make sure my pc calendar doesn't reach 2013. I have Win 7 to go back to if that happens.
You'll also need to make sure you don't ever connect it to the Internet. Microsoft licensing stuff connects to the Microsoft licensing server which sends an "authoratative" timestamp that overrules your clock for the purposes of licensing. And Microsoft also goes to lengths to stop you rolling back the clock.
Basically you're fighting the same game with untimebombing your Win8 preview as unlicenced people are against Windows8 RTM (that get a 30 day "grace" period that Microsoft goes to some lengths to stop you expanding into "forever").
If I were you, I'd make sure I back up all of my files because it's gonna get awkward when your machine suddenly dies and you can't get at them anymore.