It is possible to cast from an xs:duration to an xdt:dayTimeDuration. The
prinicile of type Substitutability therefore demands that it should be possible
to cast from any subtype of xs:duration to an xdt:dayTimeDuration. However, the
casting table in F+O section 17 says that casting from xdt:yearMonthDuration to
xdt:dayTimeDuration is disallowed.

In my view this cast should be allowed and should return a zero length duration.

The same also applies of course to the inverse cast.

(Problem revealed by running the test suite.)