Data Validation Excel 2007

On the Validation Criteria under the Data Validation option, we can choose the Date option in order to define which dates are valid.
e.g. dates from 01/04/2011 to 01/04/2012
How can we choose a specific number of days as a notice period?
e.g. Let's say Someone needs to fill in the form and put the date that he would like his product to go instore. But he needs to do that at least 28 days in advance.
What I would like the form to do is NOT to allow anyone to raise the request for his product to go instore with less than 28 days notice.

The professor in the course said that this is possible.
Could someone help me please?

MAny Thanks
Rosina