Yeah, I had that once on a 6 night ShortStay Getaway. The checkin on Marriott.com was actually the day prior to what the II confirmation showed. If I recall my experience and if this is still how it works, you will likely get seven Elite Night Credits for the stay, but don't try to checkin early. The resorts do receive something from Interval International that indicates you are only entitled to the number of nights you booked through the ShortStay. I really don't think II does anything with the extra nights because then two people would have the same confirmation number, but if you checkin early, they will check you out early. You may be able to get into your room early on your arrival day though as they may already have the room ready. It shouldn't be necessary to call the resort about arriving late ad they will have a room for you. They can't give it away or consider you a no show.