Re: expiration date for the filters on the mask.
The filters have a seal on them. As long as the seal is unbroken they'll last decades at least. If it's your mask, the seal can be broken and ya'll can practice fitting and breathing in fresh air w/o loss of capacity. Just put the cap on tight when you're done.
The active component in the mask is activated carbon. If you're handy with taking things apart and putting them back together, or improvising...activated carbon is regenerated by boiling in water for say ~30 min for most things.