Sum items in column variable relative to date - specifically getting paid on 15th & 30th

So I have a client that wants me to create a payment schedule relative to paying on the 15th and 30th for support.

The problem is that contracts end on variable dates, and different months have 28, 29, 30, and 31 days, all of which I need to be paid for.

Attached is my spreadsheet, with a RED box for the column in which the formula should be expressed for the column.

I am probably over-thinking it, but without having to manually do the calculations, I'm sure there is a more elegant way.

THANK YOU!!!

Havens