The datetime functions were very specific at only accepting variables
(backed by a table column), using the :localDate and :localTime
decomposed fields, specific internally to datetime properties.

So, actually make it able to accept other than variables (we will still
use the decomposed local time for these), and translate these as a
primary expression. This makes all these queries work (as opposed
to just the first):

select year(?date) { ?u nie:contentCreated ?date }
select year(now()) {}
select year("2015-07-29T23:00:00Z"^^xsd:dateTime) {}
select year("2015-07-29"^^xsd:date) {}