sparqlEscapeDate omits timezone offset part

[MikiDi]

Omitting the timezone offset can be interpreted here as "effectively changing the timestamps' value", since timestamps with no timezone offset specified are assumed to be local .
for ref http://books.xmlschemata.org/relaxng/ch19-77041.html

Similar mistake was made and corrected here. Note though that in the referenced implementation it is about client-side serialization of a date, while here it's about server side-serialization. Given the shortcomings of native JS Date localization options and the fact that servers are mostly configured with an UTC timezone, the approach used is in the transform-helpers, probably wouldn't suffice here.