Easy way is to have text file inside the remote folder and keep writing the count every day and check for the count in each log in and after 30th day block the application. Need to take that text file should be encripted and should not be easily accessable to the user. For more ideas please share your application details like Is it offline tool? or connects to DB over network etc.
Include a text file that keeps the date during the installation. Each time the person logs in, Check the System date against the date in the file (Installation date). If the difference is greater that 30 days, exit the app.
pgmer , sir i mean if a person change the system date of his computer then ? the agbenaza idea i think wont work.
and if you are make a encrypted file then i think you can just count the number of application is run by the user if you have limit of 30 times and user use this application 30 in a week then :P ? i think the best way to use windows registry for this .you can further visit the link i posted in my above post , you can better understand it :)