That would be the mill date.Is that the actual manufacture date? Or best before 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 guess the same science applies to horse feed.I have had feed here for the horses over winter and it’s fine when I get back in the spring. In a sealed container it will last for months. As long as it doesn’t get wet and is stored in a sealed container it will be good.