[CentOS] log time formats - where is this defined

One of my servers is using ISO datetime formats 
(2011-02-27T15:22:15.519857-05:00) in the logs
the rest use the default redhat/CentOS format (Feb 27 15:10:21).
After a couple of hours searching google I cannot find where this is 
defined.
I know I changed it some months ago as an experiment but forgotten where 
this was done.
the ISO format breaks logwatch - thus I need to revert.
TIA
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