major technical: dateTime arithmetic

Table 11.1 Operator mapping
The functionality for manipulating xsd:dateTime values is inadequate.
In particular, the XQuery functions fn:current-date and fn:current-time
are needed to formulate queries relative to the current time.
In addition, dateTime arithmetic using the XQuery types
xdt:yearMonthDuration and xdt:dayTimeDuration will be highly useful.
I suspect the reason this has not been adopted is that these types
are not accepted by RDF.  I hope that was just a historical accident
(because RDF was ready for adoption before XQuery).  However, while
we wait for duration types to be added to RDF, you can still add them
to SPARQL to permit dateTime arithmetic.  The limitations would be
that you could not use CONSTRUCT to construct a triple with a duration,
and there would be no point in trying to match a duration.

Fred