Sum of one field based on date in another field

[Guest]

I am working on a property management database. There are approx 150
properties. I've created a report which shows the financial information for
each location. I'm trying to create a few calculated fields in the report
footer, one of which I can't figure out. I would like to sum the total
square footage of properties which opened prior to a certain date. The
fields for this calculation are [Square Footage] and [Outlet Open]. I'm not
sure where to begin with this. Please help!!!

 

[Guest]

You can use a text box with a control source like:
=Sum(Abs([Outlet Open]<#1/1/2006#) * [Square Footage])