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Don't know if I should even throw this out there, since it's so vague, but...
If I recall correctly, you put something in the doc which is activated and
"datestamped" the first time the user opens the file (or executes the
program). It has a specified lifespan, which is terminated when the user's
computer reaches one month (or whatever) from it's first use. At that point,
something integral is rendered inoperable, sort of like taking someone's
distributor cap to keep them from driving away. As to how this is done, you'd
have to find a programmer to tell you that.
However, if your doc is something that will be used a finite number of times,
it might be easier to put in a counter set at 10 times, 100 times, whatever.
These counters can be reset, but not everyone knows how or will bother.
