If the user clicks Cancel, a zero-length string is returned. You can’t differentiate this from entering an empty string. You can however make your own custom InputBox class…
EDIT to properly differentiate between empty string and cancel, according to this answer.
Your example
Private Sub test()
Dim result As String
result = InputBox("Enter Date MM/DD/YYY", "Date Confirmation", Now)
If StrPtr(result) = 0 Then
MsgBox ("User canceled!")
ElseIf result = vbNullString Then
MsgBox ("User didn't enter anything!")
Else
MsgBox ("User entered " & result)
End If
End Sub
Would tell the user they canceled when they delete the default string, or they click cancel.
See http://msdn.microsoft.com/en-us/library/6z0ak68w(v=vs.90).aspx