Consistent representation of datetimes and use of calendar attribute

[pvanlaake]

See issues #624 and #625 for discussion of these changes.

Release checklist

• history.adoc up to date?
• Conformance document up to date?

For maintainers

After the merge remember to delete the source branch.
Tags are set at the conclusion of the annual meeting; until then, main always is a draft for the next version.

[pvanlaake]

All comments/suggestions to date have been resolved. Thanks to @JonathanGregory and @larsbarring for their contributions. Inviting others to review and comment.

[JonathanGregory]

Thanks, @pvanlaake. I don't think you've changed the time-zone symbol to TZ yet, which you preferred in #625. That seems fine to me: both T and Z individually are problematic, but together they make a solution. :-)

[larsbarring]

Yes, TZ is a good solution.

[pvanlaake]

Thanks for flagging that @JonathanGregory, changes have been made in chapter 4.

[DocOtak]

What ever was done for the __Z__ to __TZ__ change did not actually update the pull request. It will still be __Z__ if merged.

[pvanlaake]

My naive editing of my own PR... It should now be fixed. That is, if I preview the file I can see TZ consistently, as well as in the diffs

[DocOtak]

Yep, looks fixed to me now!