If your object in limiting XP per day is to prevent players from "harvesting" a particular enemy or area for XP, you might be interested in using a scaled XP system.
Download the Home Town (classic Diablo) single-player module from this web site:
http://sites.google....gnortheast/home
The XP system in there automatically reduces the amount of XP gained by the player(s) based on the XP already gained.
With this system, players don't have to level up in order to trigger a reduction in XP. The system uses the nominal level based on the player's XP rather than on the actual level the player has taken. This makes it cheat-proof against players not taking levels.
If the player's nominal level goes high enough, they eventually get no XP from the kills.
The whole idea of course is to force players to move on to tougher monsters in order to get decent XP.
I hope this is of use to you.