Returns the xs:date that is a given duration after a specified
               xs:date (or before, if the duration is negative).
xs:date and an
            xs:yearMonthDuration value. op:add-yearMonthDuration-to-date( $arg1 as xs:date,$arg2 as xs:yearMonthDurationxs:dateThe function returns the result of casting $arg1 to an
               xs:dateTime, adding $arg2 using the function
               op:add-yearMonthDuration-to-dateTime, and casting the result back to an
               xs:date.
The expression op:add-yearMonthDuration-to-date(xs:date("2000-10-30"),
                  xs:yearMonthDuration("P1Y2M")) returns xs:date("2001-12-30").