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

On the first start, you can store the actual date somewhere.
Each following start, you look for the stored date, if it exist you read it an if it is more than 30 days after the first start, you stop the program.