ben@nagios1:/var/log/nagios3$ grep 'mycheck-or-host' nagios.log |perl -pe 's/(\d+)/localtime($1)/e'
The perl snippet will translate the epoch-style timestamp to one that is readable.
ben@nagios1:/var/log/nagios3$ grep 'mycheck-or-host' nagios.log |perl -pe 's/(\d+)/localtime($1)/e'