The duration data types

All,

Why is the xs:duration datatype not part of XACML, while the two 
restrictions on this data type, xs:dayTimeDuration and 
xs:YearMonthDuration are?

Should we include xs:duration and the function ...:dayTime-add-duration?

It seems strange that currently it is possible to add a year and a month 
to a datetime, or add a day and a time, but you cannot do both at once. 
Was there a reason for this?

Regards,
Erik