Coordinates model - Working draft.

[Graham, Matthew J.]

The ISO 8601 spec is very clear: "The	zone	designator	is	empty	if	use	is	made	of	local	time	in	accordance	with	4.2.2.2	through 4.2.2.4,	it	is	the	UTC	designator	[Z]	if use	is	made	of	UTC	of	day	in	accordance	with	4.2.4	and	it	is	the	difference-component	if	use	is	made	of	local	time	and	the	difference	from	UTC	in	accordance	with 4.2.5.2.” If you don’t use the zone designator as in the standard then it’s not ISO 8601.

— Matthew