raydan raydan:
I have some powdered skim milk that I bought for some bread recipes that has a best before date of September 2011... I'm still using it.

You can probably use unopened packages of it for a few years to come without any worries.
Did you know that unopened and properly sealed tins of food that are over a century old can still be used as food without health concerns?
I wouldn't vouch for the quality, but it's still perfectly edible for nutrition.
It was in Napoleonic times that they found that hermetically sealed tins of food could last for a long time. That's why Arctic explorers like Franklin carried large stores of it. They just didn't know that the lead used to solder the tins shut was toxic.