Is it reasonable to assume that infirmaries would label their medicinal supplies with expiration dates to indicate when the preparations need to be used by?
Or is this a bit too modern for the Pernese? ("How's that numbweed salve?" "Smells okay, I think." "Ok, give it here.")
I think they'd either label when it needs to be used by or when it was made. Either way the healer should be able to make a judgement call from that.