What pattern applies for log4j2 configuration of %d{DEFAULT}{UTC}

My log4j2.xml has PatternLayout like this:

[%d{DEFAULT}{UTC}] ..

resulting in

[2016-08-08 22:24:58,668]..

What pattern would be used in
codec > pattern
grok > match > message

Sorry if this is a RTFM question but my search did not produce anything worth trying.