Don't make your software dependent on the system clock because people can change its date, instead put a identification on the computer so that even after a reinstall it stays, that way your software can know x number of days have passed.
I would recommend making 2 versions of your software trial and full version. Don't put all the features in the trial version (So even after doing all the tricks he / she can't access all the features!). When your customer has tried the trial version and is happy he can pay and download the full version

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