Creating a formula

Hi,

I'm trying to make a formula that will calculate and update a depreciation expense. I have a start date to each piece of equipment and an end date. I want it to depreciate every month, for 3 years, ie 36 periods.

Is there any formula I can use that will take my start date in months, calculate how many months it has been until todays date, and then multiply that by my monthly depreciation?
If so, is there a way to cap it at 36 periods? I attached a small sample so you can visually see what I am talking about.

Thanks in advance

-Tim