How to change the default formatting with boost::log::BOOST_TRIVIAL_LOG?

boost::log looks really powerful. It offers a BOOST_LOG_TRIVIAL macro for trivial logging. But how can I change the default formatting? It prints the timestamp by default, by I don't want it. Do you have any idea? It seems the only way is to define a new sink ex-novo and add it to the core, then you can call set_format() on the backend in case. But this is no "trivial" anymore.