Trial version developement

[pistol350]

Hello guys,

I'm searching for a solution to implement a trial version for a software i've developed.
My main purpose is not to "protect" the software as all the users have purchased a licence for it.
My goal is rather to automate it's deactivation after a given period of time so that the users contact "me" for reactivation.

Now, my main problem is to make sure I know that the user changed the system clock.
This can be done with very simple tests but, let's consider this example :

Let's just say that the user has not used the software for more than 6 month,
We know the last time the soft was used, but suppose 6month after,
the user decides to shift the system time backward to 5 month before launching the software.
This would result in the user winning 5 additional month of use...  
How do you solve that problem ?

Can anyone suggest a pseudo code or point me to a website for more information ?
Thank you.

[Copex]

1, when the program loads get it to check the time and date on via internet time or network NTP server
2, create a second program call it myprogram updater get it to start when the computer starts and write a time stamp to the registry either close it or leave it running checking the time every x hours, 

[pistol350]

Solution 1 won't work as the computer may not have access to internet for different reasons (technical issues or restrictive privileges issues, etc...)

Solution 2 could be interesting so i'll dig it and try to find a flaw in it 

Thank you Copex.

[Copex]

all solutions are flawed, you have to implement them all in the hope one catches them out :-)

some commercial software writes to the boot sector of the hard drive as it is inaccessible to most users, the flaw the section of the boot sector is used by some multi boot setups when running linux & windows.

you have to keep in mind most people would not even think about turning the time back, and any one that would may be able to bypass any methods implemented.