Trouble with Dates!

I've got two different kind of dates that I need to marry, somehow/someway.  

One date type is a real date associated with a sale:  the day of the sale dollars.

Other date type is a period/year associated with revenue:  the period(s) where revenue was taken against the sale dollars.

I've got each into a date format, the first was easy because it's a real date, the second I took a text version of the datevalue ( period & "/15/" & year ), but the problem is marrying the two.

I need backlog at a given point in time, which is simply:   sale dollars - revenue taken = backlog  

Basically, on a project by project basis, I get the correct answer in total, but don't have a time phased view of backlog by month.  When I run all data together, I don't get good results.... 

Help :D