Freeze dried and flake foods start losing nutrients the very first time you open it. Vitamin C is VERY unstable and will be completely gone within 3 months. That's assuming you are replacing the lid and storing the food in the freezer in between each use. Many of the B vitamins are unstable as well.
This is the reason aquarists supplement the vitamins and HUFAs with Selcon, Vita-Chem, GVH, etc.
If your dried food is older than 3 months old it should be tossed so if it's beyond it's expiration date, it will feed your fish but it's not giving them nutrition.