I realize this conversation is getting a little old - but the posts I've quoted are worth thinking about.
Milled feeds begin losing vitamins and nutrients starting the day they are milled. Grains that are not milled are good for at least a year, many are good for several years. I've read of grains being removed from the pyramids, thousands of years old, and germinating. So, in the very best of all possible worlds, you would purchase the whole grains, and mill your own feed at home every month.
So, "feed" for thought - if you can find a whole grain feed that your birds like, you won't need to worry so much about expiration dates.