no time zones on date/×? they should be added.
Anyone saying “UTC” is wrong. Unambiguously wrong if offsets are supported, and in foolish contexts like this where offsets are not supported, still wrong due to common sense and custom.
If there is no offset, there is no offset. It’s what is commonly called a naive or plain datetime. How it should be interpreted is explicitly undefined if offset-capable, and implicitly undefined by strong custom if not offset-capable; but it will generally mean in the local time zone, whatever that is—and it could be relative to a particular machine or a particular user. This is often suitable for social use, but completely unsuitable for machine history-recording use.
So: the question is rhetorical, unanswerable, thereby demonstrating why nmz’s position is unreasonable.
(Actually, only probably unreasonable because nmz’s wording wording with its “X” and “y” is not clear and may be using the term “timezone” subtly—the trouble is it’s used to mean three different things: firstly and most properly, a name for a set of rules about which time offsets to use when, e.g. “Australian Eastern Time” or “Australia/Melbourne” as it’s called in the IANA Time Zone Database, which roughly means AEST (+10:00) for half the year and AEDT (+11:00) for the other half, but conveys the rules as they have been through time; secondly, a somewhat less correct colloquial usage, a named time offset, e.g. “AEDT” or “Australian Eastern Daylight Saving Time” for +11:00; and thirdly, fairly clearly into the realm of misuse but still very common, a time offset like “+11:00”. If nmz was using the term “timezone” more precisely to mean one of the named concepts and expressly not an offset, then yeah, times written that way do require memorising a whole database, whereas offsets are straightforward to calculate, though it’s definitely harder having to do two calculations than the just one if it starts at UTC.)
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For the rest of your statement: for times not tied to a particular location or time offset, you should always use UTC in the form of the offset Z, in ISO 8601/RFC 3339 terms, since specifying any other offset indicates that it means something. (Note that RFC 3339 tried to have -00:00 be the neutral offset and Z and +00:00 meaningful, but that is acknowledged to have failed, and so https://www.ietf.org/archive/id/draft-ietf-sedate-datetime-e... is updating it to match actual usage.) But for things that involve humans and are anchored to a particular time zone, using UTC and not storing a time zone is wrong: you should store the relevant time zone and (fallback) offset so that if the time zone definition changes (as they do, sometimes with less than a few days’ notice), future times can be corrected, which they can’t be if you anchored them to UTC. So: things like system logs, use UTC; online conferences, use UTC; location-bound conferences, use that location’s time zone; general user calendars, use the user’s time zone; calendars for companies that straddle time zones (or people that work across time zones): deliberately choose a time zone or offset to anchor things to (sometimes at the level of individual events), especially for the sake of recurring event periods if you use a time zone with DST.
While a datetime without a timezone isn't terribly useful, distinguishing it from a datetime and timezone designation pair is the only correct type system.
Chesterson's Fence (https://en.wikipedia.org/wiki/G._K._Chesterton#Chesterton's_... is a very powerful design principle. They chose to put those elements into ISO 8601 for principled reasons: they come from pain. They embody responses to mistakes that I've made, and thousands of other engineers before me. Unless we fully understand the reason they were included, don't arbitrarily to do "I haven't used it, so it must be useless."
Other than that, it looks like a clean spec, but I'm not personally convinced that it has enough incremental value over JSON or YAML to replace them in the human-readable exchange format space. It can be a little more concise, but if I'm making something for humans, clarity (typically) has more value than conciseness. Are there other compelling values that I'm missing?