I have some old ones laying around. Couldn’t find the box or any info online as to whether they expire over time or not. The brand is Crest Whitestrips Renewal if that matters.
I wouldn’t recommend using them. Do you know when you got them? They won’t work as well as fresher ones. I would throw them away and buy new ones. I know they are expensive but getting new ones are better to use. 
Sorry if this is OT, but do they really work? It seems like they wouldn’t get between your teeth very well.