Look at it this way when walking: when you get off the phone with MS on any day you are changing the reservation your designated date of arrival after you hang up can be no more than 11 months from the the date you are calling AND the the designated date of departure can be no more than 11 months plus 7 days from the date you are calling. Thus, the only way you can possibly make a reservation today (Dec 19) that includes days after Nov 19 is one for which you have Nov 19 still reserved and the date of departure can be no later than Nov 26. Thus it would have been impossible today to call and drop Nov 18 through 21 while retaining any days thereafter. When you call tomorrow, Dec 20, you can drop Nov 19 but you still must keep Nov 20 to maintain any part of the reservation for any days after Nov 20 and tomorrow you will also be able to change the departure date to Nov 27 but not beyond that.
Thus, on Dec 19, today, you cannot possibly have any reservation after getting off the phone with MS where the arrival date is Nov 20 or later or the departure date is Nov 27 or later but you can have one where the arrival date is Nov 19 and the departure date is any day from Nov 20 to Nov 26.