Thank you for your comment on serializing datetime values.
As noted in the response to your comment on "Serializing xsd:decimal, xsd:float, xsd:double", SPARQL reuses the body of work for XSD and XQuery/XPath functions and operators. The rules for operations on datetimes derive from that work and this includes comparing datetime values. Support is also now to be found in many programming languages.
An implementation is free to provide custom function that converts between timezones; XQuery/XPath does not itself provide such a function.
The working group is not planning to make any changes in this area.
I would be grateful if you reply to this message to confirm that the working group has responded to your comment.
Yours, on behalf of the SPARQL Working Group,