I think the expression builder is an under-used tool in Access. Microsoft has put up a demo that shows what it can do. See Demo: Use the Expression Builder.
"You can use expressions throughout Access — for example, to calculate values, act as query criteria, or validate data. The Expression Builder helps you create expressions by providing easy access to the names of the fields and controls in your database, and to many of the built-in functions that are available to you when you write expressions. This demo shows you how to use the Expression Builder for one common task —calculating a date value on a report."