How to calculate past rent due in months

I am trying to figure out how to calculate how much money tenants owe after being X number of months late in rent. I have attached an example spreadsheet called "Monthly Rent Chart Samples"

Apt. #1 - Mary has paid her rent through 12/31/11 and is 1 month late. I did put in the formula for today's date. I then tried using the formula =DATEDIF(E2,F2,"m") however that does not work in two out of the three apartments. Apt. #1 is to be considered 1 month late since she has not paid her rent for January, even though there is not one month's time passed between 12/31/11 and today (1/20/12).

Apt. #2 - Jane is paid through 1/31/12 and is not late.

Apt. #3 - Karen has paid through 11/30/11, so she owes 2 months rent for December and January ($750 x 2 months = $1500)

Any help would be much appreciated - thank you so much!

Would this work?

=IF(F2>E2,ROUNDUP((F2-E2)/30,0),0)