I don't know that you can measure date as a numerical value. Maybe you can though. I suppose you could do that somehow if you could measure date as a numerical value.
You would have to do something like this to the database:
Code:
alter table owned_adoptables add column leveldate varchar (30) ;
Then you would when the adoptable is created edit adopt.php to store the date as a numeric number, such as 1-1-2009 becomes 112009. Then when you level up you would get the current date and check it against the one in the database and only do the level if it matches your criteria. I suppose it can be done quite easily.
I wouldn't start on this yet though as a new release is right around the corner. :)