My day job is keeping the KC10 fuel tankers flying...we use Proseal by the truckloads. A month is nothing, and I have used "expired" sealant that is 2 years past date with no issues.
I know of someone (we won't say who) that has 10 *year* old Desoto brand sealant (kept in his freezer since around the expiration date) that has been used recently to successfully repair leaks in 25+ year old tanks.
Not telling you to do as he did; just a data point.