In T132220 we found the DateTimeInputWidget uses 'Z' to denote the time selected will be in UTC. It's much more common to use 'UTC'. Can this functionality be added? and likely made default?
Nothing to do with OOUI, this is a MediaWiki widget.
(Also, "Z" is the Unix standard for UTC, it's being put their by your computer, not Wikimedia's. ;-))