ok i've done this in the past.. can't remember exactly how it's done but if you look through your date functions you will see that to add 1 day to your date you simply +1 so put in an if statement (use code builder) that goes something like;

IF inputbox1.date < (date+1) THEN msgbox("please enter in a valid date"), inputbox1.value = date

yes i know that's not gramitically correct as havn't done access for a while but I hope you get the idea?