There wouldn't really be a reliable way to tell what date you were doing stuff offline. If there is no connection to LIVE and the console is powered off completely the internal clock resets to November 2005.
Could you move the internal xbox clock to street date?
Even without a valid timestamp, the 360 stills records all sorts of other data, including DMI, PFI, security sector, capacity, and title ID, to non-volatile system memory. While this data wouldn't be sufficient to detect an original copy of a game being played offline before release (assuming the player doesn't log in until after the game's release), it is more than enough to elicit a ban flag for a console that's played a pirated game. This is the main intention of this security measure, after all.
I should also add that they're able to distinguish pirated games from originals prior to release because the scene doesn't release 'stealth' data for pre-release games, so the aforementioned data being sent to Microsoft is very clearly not authentic.
Microsoft bans people just because they fucking can.
Microsoft bans pirates 99.99% of the time. It would be bad for business to consistently do otherwise.