Apparently the System.nanoTime() is better suited than System.currentTimeMillis() for the relative time measurements done in logback, plus hopefully the resolution is finer giving more reliable figures for small operations. Apparently the currentTimeMillis resolution is 15 ms under XP where the nanoTimer is much finer. Under OS X both have a resolution of 1 ms.

As logback requires Java 5 where this is standard, it would be simple to make the switch.

Note: http://www.techper.net/2008/08/10/systemcurrenttimemillis-systemnanotime-and-their-resolution/ lists that it may be relevant to create a factory for this instead of just hardlinking to the java runtime so it is possible to link to another time source:

"A more robust timer with a 1msec resolution is LWJGL's Sys.getTime which uses TGT (time get time) on Windows and currentTimeMillis elsewhere."