180 days from each day of your vacation.
The advantage to the + 10 is that while onsite/offsite begin on the same day, onsite is allowed to go out up to 10 days from there, but offsite has to wait until the next morning for the 2nd day of the visit. Meaning, onsite could grab the really hard to get reservations before offsite is ever allowed access to those days even though the reservation time frame is exactly same.
It might not sound like a big deal, but for the difficult reservations it initially gives onsite the upper hand. Since I find people are constantly in flux for dining and such, alot of the times you are able to get what you want even at a later date; as long as you are willing to put in the time it takes.