That is true, but when I say a week vacation I usually mean the weekdays plus the 2 surrounding weekends. In out case, we are staying 8 mights, which gives us technically 9 days that we could theoretically go to the parks. We are only choosing to go to the parks only on 4 of them. If by chance we wanted our first use to be arrival day (we don't, but we could have) we would not be able to use the tickets the last 3 days of our stay, and our flight on the last day is 9:20 PM so it wouldn't be unreasonable to go that day.
We are onsite, not a package, but it's a 4/4 split stay which complicates it, so I've been asking if we would have the 7 day window or 14 day window with the complicating factore of being room only and a split stay. I don't think we know the answer yet.
I also want to add we have a combo of APs and tickets we already bought, so this is a theoretical question for future trips, for this one we are all set with old tickets with the 14 day expiration period.