Ok. You're most likely not very amused by my last answer so let me tell you what I did.
1) placed a textbox called txtMyDate on a form.
2) placed an option group called Frame7 on my form.
3) placed two option buttons on this option group (names irrelevant) and made them locked.
4) in the OnExit event of txtMyDate I typed this code:
Me.Frame7.Value = IIf(IsDate(Me.txtMyDate), 1, 2)
this asumes that used is first and unused is second. It also asumes that if something is entered in the field then a valid date is used, anything else is unused.
If you wanted to have more than two options you would need a case statement or some such thing but the theory is the same.