@Aritronix you might be thinking of a different thing that expires on September 29: the Identrust certificate chain.
This is different from your individual site's certificate which, as @rg305 noted, is only valid for 90 days. The expiration notice you received suggests that your individual certificate isn't being automatically renewed, or, as @griffin described, that you made an intentional change to the certificate's coverage (in which case the notice you received might not be relevant to you).
Edit: a more up-to-date link about what is happening in September, compared to the one I shared above, is