Also just to clarify, yes I can count back 7mths/11mths but when there is something very specific that you are hoping for and throw in weekend days or holidays and then the fact that 7mths is not really truly exactuly 7mths when you factor in 30 vs. 31 day mths to get the exact day to begin booking it almost becomes a science.
The rules at the end of the month can be confusing. The actual rule DVC uses (although even some MS CM's don't know it) is that if the corresponding checkout date does not exist on the 11/7 month call date, then you use the last day of the month, not the 1st day of the next month.
Using February as the best example, suppose you want to make a reservation at 11-months with a checkout date of Jan 30, 2008. The corresponding 11-month date would be February 30th, 2007 which is a non-existant date. So in this case you can call on Wed, Feb 28th which is the last day of the month.
Now if some year, Feb 28th would be a Sunday, then MS would be closed, and you would have to wait until the next day (Mar 1st).
This can occur whenever a corresponding date doesn't exist. Thus for any checkout date on the 31st of a month, if the corresponding date is in a month which only has 30 days, then you call on the 30th.
Of course this only occurs in these rare instances where there is no corresponding equivalent date in the 7/11 call-in month.
In the overwhelming majority of cases, users will have check-out dates on other than the 31st so it's only necessary to count back 7 or 11 to get the corresponding date. Thus if someone was checking out November 10th 2007, the corresponding dates to call MS would be December 10th 2006 (for 11-month windows) or April 10th 2007 (for 7-month windows). If either of those dates fell on a Sunday, the member would have to wait until the next day, Monday.
The exception of course is January dates with a checkout of the 29, 30, or 31st in which case at 11-months you call the previous Feb 28th (or Feb 29 in a leap year). At 7-months, only Jan 31st is special because you could call the preceeding June 30th as explained above.
DVC-Planner does use the above forumlas and also corrects for Sundays (it use to be weekends, but now that MS is open Saturdays, the program knows this), or holidays when MS is closed.