Incorporating shareware restrictions in C++ software

Question

I wish to implement my software on a shareware basis, so that the user is given a maximum trial period of (say) 30 days with which to try out the software. On purchase I intend

Answer 1

Also watch out for the time changing radically, if the current date is magically less than the install date and such.

One way to get around this type of datelock is to change your date before you install to be years in the future. So you should check that the date today is not less that the install date.