Angie,
At it's most basic there is actually the functions within InfoPath 2010 to help you build this query.
On your Due Date field (control type date). Go to Control Tools>Properties>Add Rule>Is Before>Set a Field's Value.
Fill in the boxes, .i.e
DueDate is before another date picker. Field > Date result text field. Value = yes.
You'll need one rule for yes and one rule for no.