Don't worry about it -- you're fine. A sorbate vendor stated the product is good for 12 months, longer if properly stored, although the fact sheet didn't describe that. From other research, I discovered that is needs to be kept away from air and light, at a stable temperature.Obviously none of this is ideal, and if I'd known then I wouldn't have risked it
I keep the packets for 12-13 months after purchase, which allows for the kit vendor's manufacturing timeline, and sitting on the shelf in the LHBS, etc.