Happy holidays from me, too!
I think there's a way to figure out how many days your dynasty takes. The first time a sim that was a YA at the beginning of the game becomes an elder, the age is given as 100. Since they get 20 days as a YA and 20 as an adult, this is obviously 60 days off. As your founder continues to take ambrosia, the day counter doesn't reset. Therefore, when your dynasty comes to an end, you should be able to subtract 60 from the age of your founder and get the number of days that you've taken.
Of course, that assumes that you won't have to re-start, or that a subsequent patch won't throw a spanner into the works.