Formula for monthly amount increment in a cell based on another cell's date

Hi all,

I have a spreadsheet on which I need to generate automatic amount increments on a monthly basis. Say, I have a house on rent, and the lodger may miss the due date for settlement. From that point, I need the amount to be paid to automatically increase as we will be entering another month.

In my illustration spreadsheet, you will see column B (for start dates), column C (optional monthly delay calculator), and column D (Amount Due), and this is where the formula should be entered to add, say $500 every new month.

NOTE: I could have used simplified my exercise by multiplying $500 by the number of months displayed under (Monthly Dalay Calculator). But I need to know if the formula exists based on start day. This would be more elegant, I guess!