There have been some great replies on this thread, so thanks everyone. However, going back to my original question....if the dates weren't intentional, then it would have to be an Ang Lee mistake, and I'm torn b/t thinking it was a strange twist to the movie, vs thinking that our master filmmaker could make such a huge error. Then WHY go to all the trouble of making sure that the dates were clearly mentioned....why in the fireworks scene do you HAVE to announce the year, why in the rodeo scene do you HAVE to announce the month, why does he HAVE to put the date on the postcard? Just seems those details were put in for a reason, and they wouldn't go to all that trouble if it were a mistake. Yes, lots of storyboards.....but give these guys some credit..... timelines are one of the principal things that are followed meticulously.
I do remember reading, though, I believe in the Time magazine article of several Oscar nom directors, that Ang said that he doesn't use storyboards.