clock_time_conversion serves as a trait
which can be used to specify how to convert
a source 
time_point of type
time_point<SourceClock, Duration>
to a destination 
time_point of type
time_point<DestClock, Duration>
via a specialization:
clock_time_conversion<DestClock, SourceClock>.  A specialization of 
clock_time_conversion<DestClock, SourceClock>
shall provide a const-qualified 
operator()
that takes a parameter of type 
time_point<SourceClock, Duration>
and returns a 
time_point<DestClock, OtherDuration>
representing an equivalent point in time
.OtherDuration is a 
chrono::duration
whose specialization is computed from the input 
Duration
in a manner which can vary for each 
clock_time_conversion specialization
.  A program may specialize 
clock_time_conversion
if at least one of the template parameters is a user-defined clock type
.