its easy, the first time your program runs you just have to make a file with written the date. Of course the file should have a bizarre name as libbootsrc.txt and should be hidden somewhere in the system folder(possibly invisible). Then every time your app is run you read that file and verify if 30 days have past. There is always the risk that the user alters the system time, so you should also put a "30 uses" limitation, or better keep just that limitation.
30 day evaluation
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