A simple solution would be to store the dates when the application was installed, first run, and last run. From there, you will have some basis when the system date has changed. For example, if the system date was earlier than the installed date, this should raise some suspicions.
Below is an article about it. Its written in VB.Net but it should be pretty easy to understand.
http://www.dreamincode.net/forums/topic/65721-trial-period/[
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