Nutrient loss begins to take place the moment a package of fish food is exposed to air, light, heat, moisture, or all of the above. If the expiry date is Sept 2017, it was most likely manufactured and packaged in Sept 2014.
The new fish food came today, and I hope the pellets aren't too large. I was feeding him Hikari sinking cichlid gold mini pellets before, and the new pellets are hikari sinking medium pellets. He can eat whole cubes of bloodworms, so I don't think the pellets are too big.