This is nitpicky, but it bothers me every time I read the book. I must give relief to my feelings! And I can't imagine I'm the first person to have picked up on it.
The date of Lady Elliot's death appears to be incorrect. If she died in 1800, then Elizabeth (b 1785) and Anne (1787) would have been 15 and 13, respectively, not 16 and 14, as stated on the next page. Moreover, Lady E promoted his real respectability for seventeen years, which, since they were married in 1784, would make her death actually in 1801 (which would be consistent with daughters' ages of 16 and 14 at the time).
Other references to the daughters' ages are correct (ie, novel is set in 1814 and we are told elsewhere that Elizabeth and Anne are 29 and 27, so it seems only that the date of LE's death is incorrect.
I thought perhaps Sir Walter had let something slip incorrectly into the Book of Books, but since he inserted "most accurately the day of the month on which he had lost his wife," this hypothesis is implausible.
Did they use different math back then?