The short story says that 'Ennis did not hear about the accident for months, until his postcard to Jack ... came back'. I think A Proulx meant that Jack went to LF straight after that last confrontation, mentioned Randal on the heat of the moment, and then was killed sometime in that same year (1981 in the movie) before 7 November; and that Ennis got the deceased card in late 81 and went to see the parents in the 81-82 winter, probably asap in order to get those ashes. With the autumn leaves on the ranch trees, I had always thought it was late October or ealy November, a tragic alternative to the fishni'trip. Now with that new theory of Ennis having received no answer by 7th November, I am confusing myself! I don't think Jack ever received the card, the post office printed it 'deceased' precisely because it could not be delivered to a dead addressee. This is part of the tragedy, Jack dying without ever hearing from Ennis again after their argument.
Probably because there were so many revisions to the screenplay, dates that made sense in earlier versions got confused later, and the writers overlooked this aspect as, after all, it has no huge impact on the essence of the story.