[CentOS] certificate for www.xxxx.xx has expired ?? (hostname)

Hello,

I found this Morning a Message in my Logs, that is new for me and I never seen 
this before?

Is this a Error in the Certificate System??

The certificate for www.example.at has expired
Datum: 08.07.17 06:31
Von: root <gjn+www at example.at>
An: gjn+www at example.at
 ################# SSL Certificate Warning ################

  Certificate for hostname 'www.example.at', in file (or by nickname):
     /etc/pki/tls/certs/localhost.crt

  The certificate needs to be renewed; this can be done
  using the 'genkey' program.

  Browsers will not be able to correctly connect to this
  web site using SSL until the certificate is renewed.

 ##########################################################
                                  Generated by certwatch(1)

what is the best way to correct this Problem?
-- 
mit freundlichen Grüssen / best regards,

  Günther J. Niederwimmer