[tz] [PROPOSED] Be more consistent about UT vs UTC

[Paul Eggert]

On 11/07/2017 10:01 AM, Philip Paeps wrote:
> Explaining / apologising for our ancestors seems wise though.  Perhaps 
> a more prominent note somewhere for the uninitiated?

I'm hoping that the bullet point in theory.html suffices; I mean the one 
that says "In the tz database commentary, UT denotes a family of time 
standards that includes Coordinated Universal Time (UTC)...." If this 
wording could be improved or if featuring it more prominently would 
help, specific suggestions are welcome.

Before the recent change, tzdb was inconsistent; although quite a bit 
already respected the distinction between UT and UTC, quite a bit 
didn't. The idea of the patch was to change tzdb to follow the bullet 
point more systematically.

> I wonder if it wouldn't be easier to simply note "where we write UTC, 
> we actually mean UT when referring to timestamps before UTC was 
> introduced".

Something like that was proposed more generally, even for applications 
outside tzdb, here:

Sauter J. Extending Coordinated Universal Time to dates before 1972. 
2017-07-08. 
https://commons.wikimedia.org/wiki/File:Extending_Coordinated_Universal_Time_to_Dates_Before_1972.pdf

Sauter also suggested that we drop the use of time_t for telling time:

Sauter J. Avoid using POSIX time_t for telling time. 2017-10-08. 
https://commons.wikimedia.org/wiki/File:Avoid_Using_POSIX_time_t_for_Telling_Time.pdf

If these recent proposals catch on, I suppose we could change tzdb 
eventually. Dropping time_t would be a big change, though. Even 
extending UTC for old timestamps is a stretch.