athena, if I recall correctly (I'd have to check), the timeline is slightly different in the movie than in the original SS, so it's a little confusing. I finally chose not to get too caught up on the year and to just concentrate on the months, as you asked about.
My sense is that it was late winter/early spring when they had their final trip (because Jack was complaining about the cold), and this was why Jack was upset about losing August - waiting 'til November would mean more months apart. As to when Jack died, that too is somewhat unclear. All we know for sure is that it was sometime between that last meet and November, and long enough past for Ennis's postcard to be returned and Jack's ashes to be distributed between Texas and Lightning Flat.