Though not Ziplocs, I bought some other plastic bags in the late 80s that are still good now! They were 2 types of plastic bags that I pack bread in, for freezing, and the long one was for baguettes, the other for the usual loaves I make, and I got a thousand of each for around $12 a case, from a restaurant supply shop. I still have a lot of the long ones, only a couple hundred of the loaf bags. I also got some odd sized Ziploc type around the same time, in large amounts, and never had any of them degrade at all. So even if there's a date on them, you don't have to worry about it, as long as you keep them out of light.