I'll just mention that without specifying the date with the -S option, this query/retrieve will take a very long time if your system has so many System Log tables that you're considering archiving it elsewhere.
Otherwise, Alessandro is correct again.
The only thing we haven't addressed is how to remove the System Log data from CMOD.
You can either reduce the 'life of data and indexes' value, or run arsmaint with the (rather dangerous) -t option to get CMOD to expire the data, but as if arsmaint was being run at a future date.
-JD.