How do I avoid selfcontrol to stop just by changing time settings on mac?
You could try and contact the Developer. Tell them about this get around.

Ask them if they could find a way to avoid it.

For example:

Have their app check a online clock instead of the system clock.

That works like the Time and Date System preference which uses  Apple Americas/U.S. (time.apple.com.)

Suggest that since you are able to connect to the net ( which I assume is the problem ) then access to the online clock is essential for the app to allow you to browse.

If the app cannot connect to the online clock it will not allow you to browser for a set period.

And after that period example 2hrs. It will allow you 10 minutes grace to connect to or select a different clock in your time zone.
 And then stop for another two hours if it is still unable to connect to a clock.

The normal rules will apply if it can connect within the ten minutes grace.