only allow SU (parent) user to change time interval one day earlier instead of on the same day

You can change time interval and limit for any day you want, just select the day and change it :)
If I did not quite understood your issue, please explain your use case in more detail.

I mean the application timekpr-next should maybe have a feature that forbid super-user to change time interval and limit for the current day, so the super-user must plan ahead at least one day.

can I use pam_time to restrict the admin panel " timekpra"?

Ahh, ok, now I understand what you mean :) You basically mean self-control, right?

Well, since you are superuser or at least sudo user, you can not really restrict anything... There will always be a way to circumvent that.
I can possibly make it as a preference to enable this, but you will always be able to go to config files and change that manually. As a last resort, you'll always be able to boot livecd and go change config anyway.

Is there a specific thing you are trying to restrict yourself? What happens in case of emergency, like you actually really really need the computer you locked, is there a possibility of that?

Hey, thanks, it happened indeed once that I missed a deadline since I can not access my laptop (travelling without livecd), but I never regret. Since thinking backwards, I would prefer to sleep than to work late every day. So I think it would be cool to have an extra feature to allow only plan-ahead time interval like lock the possibility to change the interval in the afternoon (which I assume pam_time will do? dont' know how yet) or on the same day.

Timekpr-next does not use anything from PAM, so that could be an external measure to prevent yourself from doing too much work :)

I really have to think about this and I'm not really sure it would be helpful, becasue if you really wanted you actually can change the config or stop timekpr daemon no matter whether it's locked in GUI, Timekpr won't be able to help much or stop you from doing that...

But you can always restrict yourself - just make a rule in your mind that you must not touch settings in the afternoon, otherwise you need to perform 100 pushups or so :)