boost::log::expressions::format_date_time
// In header: <boost/log/expressions/formatters/date_time.hpp> template<typename T, typename FallbackPolicyT, typename TagT, template< typename > class ActorT, typename CharT> format_date_time_actor< T, FallbackPolicyT, CharT, ActorT > format_date_time(attribute_actor< T, FallbackPolicyT, TagT, ActorT > const & placeholder, std::basic_string< CharT > const & format);