It depends on how slick you want to be and how many clients you have:
- if there is only one client, you could just code in an end date into your startup PRG
- each time your exe starts write the date into a table. When the current date is more than 30 days past the min date in the table. Abort the program. You would want make the table something the user wouldn't guess. (not daysleft.dbf but instead something that is nonsense like videodrv.hlx

Also check out Craig's articles. Two deal with demo versions of software
http://members.home.net/foxpro/kb/index.htm

>If a want to send a EXE to my cliente,
>and I want to limit it with a date.
>I mean: I want my client to use my EXE
>for 30 days.
>What's the best way to keep this date?
>I don't want to use another sofware besides
>VFP 5.0