Received a notice that our R3 certificate will expire on 24 June 2021. This certificate should expire on September 29 2021. Is there any reason for it to expire on June instead of September? Can we extend it till September?
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Which "this" ?
[as yet, LE hasn't issued any client certs that can expire on September 29 2021]
[client certs are issued for 90 days]
@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