Implementation details for date:date-time ()

The implementation page doesn't define the meaning of "now".
I.e. should this function return a different date-time every time it
is called, or should it always return the same time throughout a
transformation, as in XPath 2.0?
If the latter is acceptable, then presumably I can reuse my code for
implementing fn:current-dateTime unchanged?
-- 
Colin Paul Adams
Preston Lancashire