Not a bug - Of course you're getting an error because what you have typed is text. If you want to allow things like "the second friday of july, next year" in your date field, you'll have to do some scripting behind it.
Booking are a serious thing, so you want to be sure of what you get, for example by looping the message till isValid(gDate) AND gDate ≥ Get(CurrentDate).
Helping the user to understand what you want with "Please enter the booking date as MM/DD/YY or MM/DD/YYYY" is also a nice touch.
Another option would be to use a modal window as a pseudo-dialog box. This way you'd be able to use a field with a drop-down calendar. You'd be assured of a correctly entered data. Pretty much. As soon as somebody builds a better mouse trap a better mouse will come along. Be careful with the modal window, it is possible to lock yourself into the window with no way to get out.
You might also look at something like Troi Dialog. They have lots of tools for messing with fields in dialog boxes.