It's possible to create a custom logback converter, which can format the millis. http://logback.qos.ch/manual/layouts.html#customConversionSpecifier

That is not something we can provide, since it is logback specific, but possible to document if we prefer that.

If we don't want the overhead of formatting it in Slf4jLogger we can output the millis diff to the log %date. I guess that can only be an approximation, by calling System.currentTimeMillis again.