[tz] Why is `Etc/UCT` not an alias of `Etc/UTC`?

[Isiah Meadows]

Okay. I was just under the impression UTC and UCT are the same and
thus have little reason to be distinct. It just seemed odd they were
different, especially when they're used synonymously, and knowing
tzdata is so broadly used for basically everything, I could reasonably
see a ton of people relying on the non-aliasing behavior.

The citations seem interesting, so it is definitely worth a mountain
and three fourths of testing if you were to make any changes to
`Etc/UCT`.

-----

Isiah Meadows
contact at isiahmeadows.com
www.isiahmeadows.com

On Tue, Mar 5, 2019 at 4:45 PM Paul Eggert <eggert at cs.ucla.edu> wrote:
>
> On 3/5/19 12:00 PM, Isiah Meadows wrote:
> > So basically just back-compat?
>
> That's one reason it's there, yes.
>
> My impression is that "UCT" is used on occasion as seen in the citations
> below, although it's by far less common than "UTC". The question is
> whether a shell command like "TZ=Etc/UCT date" should generate the
> abbreviation "UCT" (perpetuating the "typo") or "UTC" ("correcting" it).
> I am quoting the words "typo" and "correcting" so as not to prejudge the
> matter.
>
> Some sample uses of "UCT":
>
> Gray JE, Machlan HE, Allan DW. The effect of humidity on commercial
> cesium beam atomic clocks. Proc 42nd Ann Freq Control Symp 1988, 514-518.
> https://dx.doi.org/10.1109/FREQ.1988.27649 "The implications of such
> changes are quite significant in the generation of International Atomic
> Time (TAI) and of Universal Coordinated Time (UCT)." The authors were
> from the Time & Frequency Division, NIST.
>
> Kouba J. Relativistic time transformations in GPS. GPS Solutions (2002)
> 5: 1. https://doi.org/10.1007/PL00012907 "It is interesting to note that
> TT, TAI, and UTC (the Universal Coordinated Time) all have nominally the
> same ..." The authorwas from the Geodetic Survey Division, Natural
> Resources Canada.
>
> Messerschmit DG. Relativistic Timekeeping, Motion, and Gravity in
> Distributed Systems. Proc IEEE, vol. 105, no. 8, pp. 1511-1573, Aug.
> 2017. https://dx.doi.org/10.1109/JPROC.2017.2717980 "Universal
> coordinated time (UTC) is the common civil definition of time." The
> author is an emitus professor in the EECS dept, UC Berkeley.
>
> Freeman RL. Standard time and frequency. Reference Manual for
> Telecommunications Engineering, R. L. Freeman (Ed.) (2002).
> https://doi.org/10.1002/0471208051.fre035 "Another necessity, in some
> cases, is to be able to derive universal coordinated time (UCT) or a
> comparable time scale." Freeman's a widely-published telecom author.
>