Hi, hope you are enjoying BYC! 
 
You already got good answers... I will just add that HOW it's stored matters.
Recently I had to purchase from my LFS... so after I got I home I was dismayed to see a DEC mill date. But had to get another bag the following week since we haven't made it to the city yet (yep 50# a week right now)... and imagine my disgust when it had a NOV mill date! 

 I mean my town is small and we are just coming out of winter, but come ON. But wait there's more... half way through the bag and see little dark pieces of feed. Low and behold it's mold FROM THE FACTORY.. like they new they had it but spread it thin enough throughout the feed so it would pass inspection or something. No clumped feed or damaged bag. 
 
Long story short... I drove 90 miles each way JUST to buy feed 2 days ago. I also save at least $3/bag... and as they were loading it in, I noted the FEB mill date on the new feed. Now you see I drive a ways, and I go through a bunch... but we only go once a month (usually to see the in-law's but we've played hooky recently 

), anyways point is that my feed is almost always more than 6 weeks past mill date since my last bag will have been at my own house for 4 weeks..
More important than mill date... in my opinion... DON'T feed scratch or other low nutrient snacks excessively. This is ALMOST ALWAYS the source of issues that are feed related in MY experience. And while mill date IS important.. overall nutrition really is KEY. 

 Make it the correct protein and calcium levels for the age and breed of the bird. Formulated rations have added vitamins, minerals, and amino acids that scratch and tables scraps don't.