I see. You're looking for a layer feed. Any would technically be fine, but feeding as Sally does (above) is a better option.
They word it that way on the assumption that you will be switching to layer. The feed is fine for life.
@RoyalChick, I have heard some say six months is fine. I believe some feeds put a one year expiration. If you are inclined to trying the freezer, I would recommend a test batch to see if it draws moisture and molds. It all comes down to your judgment.
That would be the mill date.
The general concern with the chicken feed is nutrient loss over time more so than it going "bad". Pellets keep better than crumble in this regard as the pellets have less surface area compared to their volume. Likewise whole grains keep better than pellets. I would...
The date style change could be if it was made by or made for. Sometimes a company will contract a smaller mill to make feed for that area and the dating method could be different.
I always have to ask for it. 99.8% of the time they do have it - just not on the shelf. I don't know why. Middle of January was the last time I bought it straight off the shelf.