validate for a time span

Question

Hi to all, I am developing a desktop based application software. i want to make 30 days trial of this software. i don't want to use user's system date or internet date to validate this software for 30 days. i can validate this software by using the user login information and storing it in log sheet. but i need to define 30 days whether user log in or not. it will expire after 30 days automatically. How can i do this?
Please help me.....

Thank you to all in advance

With regards,
Sumit