The new value [as of
20 May 2019] of the Planck constant by the ISO standard is set to 6.626 070 150 x 10-34 J⋅s.
The currently defined value is:
/// Planck constant
BOOST_UNITS_PHYSICAL_CONSTANT(h,quantity<energy_time>,6.62606896e-34*joule*seconds,3.3e-41*joule*seconds);
Hmmm, that's not a fun problem, thinking about it [in terms of Boost releases of before and after 20th of May 2019].