2007 - Using The True False Function

This is probably quite a simple request but here goes:-


=SUM(h2,e2-f2)

I would like excel to perform this calculation but only if another cell contains a date. I couldn't figure out how to do that so I added another column next to that cell and added =ISNUMBER(A8) to return a true or false statement. I was then going to hide this column but still refer to it.

transaction cleared amount amount balance
date date in out
A B C D E F H
1 $500.00 BALANCE BROUGHT FORWARD
2 1/1/10 TRUE 1/2/10 TRUE $5.00 =SUM(h2,e2-f2)
3 1/2/10 TRUE FALSE $300.00 =SUM(h3,e3-f3)
4
5
6
7

I don't know if this makes sense of if anyone can suggest another way of working this out. New to excel so sorry if this is basic stuff.