...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
boost::units::abstract::time_unit_tag
// In header: <boost/units/systems/abstract.hpp> struct time_unit_tag : public base_unit< time_unit_tag, time_dimension, -28 > { };