How to protect Licensed softwares from system date-time changes?

Step 1: Create trial_tracker entry in an encrypted format in a windows registry and in file.

Step 2: Assign app install timestamp ( yyyy-mm-dd-hh-mm-ss ) to trial_tracker.

Whenever app starts, check if current system timestamp is greater than trial_tracker and less then expected expiry date.

•     If yes, update trial_tracker to current system timestamp and continue.

•     If no, trial_tracker has been tampered or trial time expired. Ask user to purchase full version or exit.

Note: User can get away with this by deleting windows registry entry and encrypted file.( if he is able to find them ). In such case, further checks can be added. For example create secondary windows registry entry which checks for existence of primary registry and encrypted file.