[tz] apply Portuguese decree of 1911-05-30

[Paul Eggert]

Thanks for the fixes; I installed the attached patch into the 
development version on GitHub.

On 02/15/2018 02:05 PM, Michael H Deckers via tz wrote:
>
>    The following proposals contain the necessary corrections. I have also
>    tried to indicate exactly which data are implied by the source. I see
>    this as a small step to make the tzdb data traceable to its 
> sources, and
>    thus to clarify the uncertainty of tzdb data (when they are 
> estimated, or
>    derived from less reliable sources). The format of these 
> indications can
>    certainly be improved.

Thanks, though I'm afraid that the tzdata source doesn't lend itself 
well to this kind of detailed provenance annotation, partly because the 
data for a transition is partly at the end of one line and partly in the 
start of the next line, and there's little room for commentary that 
exactly matches the transition. I installed the attached patch, which 
uses a less-formal style to mention the 1911 Portuguese decree in every 
place that uses it.

While we're on the topic of provenance, I use 'git blame' for tzdb 
provenance, and recommend it for doing meta-detective work about why 
data entries are the way they are. Although 'git blame' doesn't always 
give the info I want, it's convenient and doesn't get in the way of 
regular maintenance.

>
>    [pt 1911] specifies Cabo Verde to use UT - 02 h since 1912-01-01 
> 02:00Z.
>    Currently, tzdb has a switch to UT - 02 h in 1907 without source 
> (Shanks by default).
>    This switch would precede the one in [pt 1911] which appears to be 
> unlikely
>    (admittedly, we know from Guernsey on 1913-06-18 that such 
> conclusions may be wrong).

For now let's go with your guess, as in the attached patch.

> Note that Lisbon Mean Time as in [pt 1911] and in [Milne 1899] differs 
> from
>    the GMTOFF given here for it (from Shanks?). 

The Shanks-derived offset was wrong, and was fixed in commit 
2951fa3bbc8d68e894e96a2399322254e20714a2, first released in 2018a. I 
suppose you were looking at a pre-2018a copy of the relevant file.
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