The second one. But the word is spelled "inedible" This is inedible. It's a month past its expiration date.

It's spelt 'inedible.' The first sentence is wrong but the second sentence is correct, minus the spelling mistake. The most natural way to say something has passed its expiration date, at least in the UK, is to say that it is 'out of date.; This is inedible. It's a month out of date.

I never use the word inedible in this context. I would say, I can't eat this - it's off. It's a month past the expiry date!

inedible - note the spelling Your first sentence is incorrect - the second is fine. We would probably say "expiry" date more than "expiration" We could also say "past its use-by date" - this is reference to the words "use by" stamped on food packages and cans to indicate the consumer should "use" the product "by" a certain date.