>Yes, that's basicly what I would do, however there will have to be buffers to allow
>normal time-correction and summer/winter-time changes.
>
>Anyway it's quite a lot of handling stuff. And You will additionally have to write it
>to a file that is not easy to identify or at a hidden place in the registry, Not knowing
>whether the actual user as administrator or just "guest" rights on the machine.
>Would be much more easy if I simply could make an api-call to an internal system clock
>that cannot be changed.

I don't think there is a system clock which the user cannot change....

>Actually no. I think they are just normal users. However changing the system time
>is an idea that even the greatest computer-dummy might have.
>My application is not expensive, and I can easily live with a few hackers that
>use it for free, but I'd like at least some security.

That was a joke, I know changing the system time is a basic idea.

>Actually that's not a bad idea. It does not have to be a timer at all. I could write a
>small custom class that when instantiated catches the current time and holds this in a property.
>I'd add the control to _screen uppon application startup. In the destroy I'd catch the time
>again and automatically add the elapsed time to an entry somewhere. Even using a timer would
>barely use any resources when firing once in a while and simply adding a certain amount to
>a property. So I could internally check 30 days or 30 x 5 = 150 hours (assuming that the
>application in average runs maximum 5 hours per day). That would at least one day terminate usability.

I was thinking at a timer because in other ways you have to manage if the user quit the app killing it (with task manager i.e.) and not without 'normal' closing.
Not really difficult to do

>The question for any of these solutions is, where to store it safely. I'm not a friend of
>adding rubbish to other's people registry. And if You want to make this work even with reinstalling
>the software You will have to leave it in the registry forever or until the user buys the software.
>Same is true for a file that You hide somewhere on the system. However if You put it in Your
>installation-directory, the user might simply delete the whole application-dir and the information
>is gone.

Yes, use registry, systemdir, ecc.
we store it in the registry and all is crypted (of course)

If you think at how many cookies, temp files, ecc. we got in our pc at a day,
don't feel bad to add few bytes to preserve your copyright.