Validate Field for Date "Accept only 1 week date!"

I am designing a room reservation in my college...
The rule of the booking is, it only allows booking at most one week ahead.

For example, let say today is 09/11/2004,
You can only book for a room from 09/11/2004 to 15/11/2004. That is a 1 week range.
The date format is dd/mm/yyyy.

How can I make the form in my asp page or a field in my form
to accept only the one week period of time from the date the booking is made when the form is submitted? Any idea....?

Please help me anybody.
I cannot find out how to do that!

Is there any popup callender picker that we can select only one week range from todays date? Could anybody help me please......