I buy a lot of hops from Yakima valley hops. They're great quality, fresh and dated. After opening a bag I package them with a vacuum sealer and put them in my lagering fridge set to 32.5F.
They keep really well. The ones I use most never last long. Though it seldom happens I will use hops up to a year old(in my freezer) if they pass the sniff test. I'll adjust AA% down as they age just based on gut feel or whim which are both scientific measures around here.
I usually buy the freshest dated hops unless there's a crazy sale on the previous harvest and then it depends on the hops.