To answer your first question, yes, it would be possible, but it's going to be a hassle. You'll need to create a separate "installer" program which installs the worksheet and registers the installation date in the system registry. Every time your worksheet is opened, it can check the registry and, if the current date is more than 14 days after the installation date, deny access to the functions.
As far as making sure the spreadsheet isn't redistributed, there really isn't a good way to do this. The best approach, I believe, would be a remote registration process such as Windows XP activation uses. Think hard about whether you really want to do this -- I think this is considerably more work than you've bargained for.
On another note, I'm not sure Microsoft would approve of your distributing VBA modules (embedded in Excel worksheets) on a commercial basis. I could be wrong about that, but it might not be a bad idea to check it out.