Both of these queries subtract a little over a day from an xs:date. In both
cases the result is one day off - at least I think so. I.e. -7:

The query in op-subtract-dayTimeDuration-from-date-7 is:
> fn:string(xs:date("1989-07-05Z") - xdt:dayTimeDuration("P01DT09H02M"))

Expected result is: 1989-07-04Z

I believe it should be 1989-07-03Z as the dayTimeDuration is a little over one
day and dates are defined to be starting at 00:00:00, so subtracting a little
more than a day and discarding the time part thereafter should yield the 3rd.

Same goes for -1, which cites below example, but actually presents different
results.

See also
http://www.w3.org/TR/xpath-functions/#func-subtract-dayTimeDuration-from-date
which includes a very similar example.