Inputbox to validate user date entry

I would like to take a user input (a date) via an OK / Cancel box... so I'm successfully using an input box.  

Now I would like to make sure the user input date is:
- Not later than todays date
- If the user presses "cancel" the input box will cancel
- If the user presses OK and it's not a valid date and later than today it will prompt for an entry again.

I'm using the following code but I can't get it to check dates correctly.  If I Dim sOutRxDate as a date... it helps but then I can't tell if the user pressed escape.

Any ideas?

Dim sOutRxDate As String

    Do Until IsDate(sOutRxDate) And sOutRxDate <= Now()
        sOutRxDate = InputBox("What date was the outside Rx was issued?" & vbCrLf & vbCrLf & vbCrLf, "Outside Rx")
        If StrPtr(sOutRxDate) = 0 Then Exit Sub 'User pressed cancel
    Loop

Thank you.