A couple things
At my day job, we use Proseal by the truckloads..(USAF re-fueling tankers). I've been playing with it for 40 plus years and working direct with MFG's of all different variants. We have been given extensions on the exp dates based on how it has been stored. As others have said, it will last a looong time. 90% of failed cure is improper mixing, and always make a sample card with batch number and date to tack on the wall, especially if you are closing a tank and need to be 100% comfortable it set up inside correctly.