Hi Scott,
There's a couple pieces here... First off, I don't think you need to use an Input Text object at all. Just a Hotspot placed over the image of the input field that, when clicked, shows the calendar. See screenshot attached. Unless you also want users to be able to type in a date.
Secondly, how high-fidelity is your simulation? Is there a
correct date they
must choose? That's simple - there would be one correct hotspot, and a second "catch-all" incorrect hotspot below it. Or can they select any date, and it will fill in the field? That will require a unique hotspot for each day, and unique logic to translate each hotspot click into a different date in the date field.
Hope that helps!
- Nav