Run Php Script using access code which has a lifetime of 30 days [closed]

I have a php script that users can run when they enter a correct and active access code. My idea is to pass the necessary parameters (access code, etc) from a start page to my PHP script using the GET method. When isset($_GET[]) is executed I would like to check if the access code is correct and active, if so a function should be called. How could I implement such a query of an access code, which remains active for 30 days after the first execution?

In summary: -How can I check the 30 days validity of keys stored in a file or something similar? -How to start such a timer when using a key for the first time?

I don't need a detailed description but just some hints for the implementation.

I hope someone can help me.

Thanks