Thanks DreamerZ. I double-checked the properties as you suggested, and everything was set the way it was supposed to be. So, scrapped everything and recreated the base query, form and combo box - and still no luck. Here is some more info, to see if you spot an error.
I first create the new "select by date" form by copying/renaming a similar form (let's call it "select by customer name"

that has a working combo box. Then I change the new form's Record Source to from the "sort by ascending customer" query to the "sort by ascending date" query. Then I then delete the "customer name" combo box and create a new "date combo box" through the wizard.
The four times I've followed this process with date fields has resulted in working forms. What is weird is that if I open the "date select" form, use the navigator buttons to "toggle" through the database, and then do the date selection in the combo box, the pointer always moves back to the first record - regardless of what date is selected in the combo box. So it seems to "see" the combo box results, but doesn't take the correct action on the database.
As another angle, I also created an entirely new table with two fields (one date, one text), and created a date-sort query, a new form and a date combo box. Same results! So, it doesn't seem like something unique to my table. Any other ideas? Thanks.