Stuart, If you do one simulation per thread, then you can put what you need in a ThreadLocal, then use a Custome Converter to read from the ThreadLocal and get the SimTime. See http://logback.qos.ch/manual/layouts.html#customConversionSpecifier If that is not enough ... Would it be possible to implement what you want if (http://jira.qos.ch/browse/LOGBACK-719 "Provide a mechanism to specify an appender factory class") were implemented? I've not reviewed the associated changes, so I'm not sure, but you could have a look. If they look good and you test them out and it works for you, then that's more reason for those changes to be accepted into logback. David -- View this message in context: http://logback.10977.n7.nabble.com/Why-are-encoder-less-appenders-not-consis... Sent from the Users mailing list archive at Nabble.com.