If you’re like me and most humans, the Nagios logfile timestamp (a unix timestamp) isn’t terribly useful when trying to grep through the logs and correlate events:
# head -2 nagios.log  LOG ROTATION: DAILY  LOG VERSION: 2.0
Here’s a nifty Perl one-liner that you can pipe your logs through:
perl -pe 's/(\\d+)/localtime($1)/e'
to get nicer output like:
# head -2 nagios.log [Tue Oct 16 00:00:00 2012] LOG ROTATION: DAILY [Tue Oct 16 00:00:00 2012] LOG VERSION: 2.0