You can use form rules or validation rules, either or. I'd suggest just making a checkbox field that checks your date field against today and if the gap is less than 14 days check it off, then make a form rule or setup you validation rule when the box is checked and display a popup message/abort the save.
Thanks Chayce, one question. I don't recall seeing the option to popup a message and abort the save. Is that something that shows on Checkbox fields?
Brian
This answer is slightly different if you're on new forms or legacy - but in the 'Actions' part of the form rule there are two things:
Looks like I am on a legacy form. Is there an option to do this there?
Qrew LegendOn Legacy forms, the option is under
"When the record is saved",
There is an option to Abort the Save (with a message).
