Expiration dates are created for every product in the exact same way. They take the fastest rate of decomposition (normally at the creation of the chemical) and they stamp a date as if that decomposition rate were to continue forever. The fact is that rate drops of drastically due to a bunch of stuff that kinetics in chemistry can explain. At any rate, they put that date for legals reasons. It's a law that they put the date as if the fastest rate of decomp. goes forever. The m1t's, like many OTC chemicals are good for long past their exp. date.