What brand canisters? CS/CN cans may have an expiration date, but they don't work like that. You use them until you get breakthrough and then change them. If it was CBRN filters, you definitely want to replenish your stock when they expire. After you've replaced them with new ones, you can open and use them as CS/CN/training filters, until you get CS/CN breakthrough, and then replace them. CBRN cans must be kept in their sealed packaging, or you might as well just use them for training and CS/CN exposures, because fresh air degrades the CBRN protection in them over time.