[Specs] Permit only UTC in `<date>`
Motivation
Time zones are a mess. It is hard for computers and humans alike to work with them. Avoiding them seems to be the best option.
Suggested change
The <date>
-tag as defined by 4.1.1.2.8 shall no longer be allowed to use any time zone other than UTC. Generators MUST generate timestamps in UTC. Parsers SHOULD accept time zones beside UTC. Parsers MAY report non-UTC time zones (independent on if they are supported or not).
This also allows comparing/sorting with purely alphanumeric comparing/sorting after normalisation of UTC to "Z"¹.
Required version updates
This change will not require a version or namespace update.
¹ Quoted from the "XML schema dateTime" specification:
The mapping so defined is one-to-one, except that '+00:00', '-00:00', and 'Z' all represent the same zero-length duration timezone, UTC; 'Z' is its canonical representation.