* For format, all a type must do is implement to_stream.
* For parse, each type X must specialize parse_manip<X,CharT,Traits>.
Each specialization must include a public typedef to itself named
type.
* Each parse_manip specialization must have a stream extraction
operator.
* This commit depends on expression-SFINAE. If this commit breaks
your build, it is likely that your compiler/version does not
support expression-SFINAE. To fix this NO_EXPRESSION_SFINAE
needs to be defined in the configuration area of date.h for
that compiler/version.
* The unit tests should reflect basic definitions.
* Derivative relationships do not need to be tested.
* Derivative tests needlessly increase test times and noise.
* After much consideration I consider this not a good change.
* It introduced a completely arbitrary mathematical relationship
among the units.
* This commit restores a precise relationship among the units which
is based on the Gregorian calendar.
* However this experiment has been preserved in the git history.
* The Gregorian average of months and years is an
integral number of seconds.
* This patch slightly tweaks the average to make it
not an integral number of seconds, and thus these
units no longer implicitly convert to seconds.
* This will flag naive mixing of years, months and seconds
as a compile time error.
Howard Hinnant