I revisited the questions and what I learned about the dates in the Grantor Index in an article that is in the current MASSOG (Vol. 40 (2015-2016), no. 1, 22-26).
It is easy to say that the Grantor Index clearly gives a date of 12 August 1659, so that MUST be the date of the deed. Looking at the deed "clouds the issue" -- it is NOT dated 12 August 1659. The word "August" doesn't appear anywhere in the deed!
If you can't easily find a copy of my article in the current MASSOG (it is available online to Massachusetts Society of Genealogists (MSOG) members), I recommend you go back and read my blog and you'll understand the challenges of the date of the deed.
I will recommend that folks take a look at the whole issue of the MSOG's Journal MASSOG, it has interesting articles besides mine. Happy reading!
©Erica Dakin Voolich, 2016
The link to this page is http://genea-adventures.blogspot.com/2016/01/the-date-of-deedtruth-or-fiction.html