Restrict date update if 14 days after currently selected date

I do appreciate your help very much, but I really have no clue what you're telling me to do. I didn't understand single sentence. I don't blame you, it's probably me, because I'm newbie in all of this.

I thought your explanation will be much easier to follow.

Now I don't know what I did nor what I should do.

Is there a way for you to tell me in more basic language. Like: 1. create this field with this type; 2. create this form rule etc.

I'm confused right now...