Hi Andrew,

Regarding the software problem you're seeing, it's quite possibly a result of using a language or API in which dates -- like 2020-01-09 -- are represented as localized datetimes at midnight -- like 2020-01-09 at 00:00 in America/Regina. This is a pervasive class of bugs that affects JavaScript programs for users in Brazil and Chile in particular. JavaScript because the language's builtin Date representation is a datetime (and the ubiquitous moment library isn't so careful with it), and Brazilian and Chilean users because until recently their time zones had midnight spring forwards. Here's a particularly illuminating example of this class of bugs: https://github.com/airbnb/react-dates/issues/776.

Hope that helps you track down your software issue.

Scott Kilpatrick

On Wed, Jan 8, 2020 at 8:07 PM Paul Eggert <eggert@cs.ucla.edu> wrote:
On 1/8/20 11:49 AM, Andrew Lyon wrote:
> It appears that many use the tz database as a source of truth.

Ouch. As we say in big letters, "The tz code and data are by no means
authoritative." <https://data.iana.org/time-zones/tz-link.html#changes>

> I understand there were 10 years between 1930 and 1941 when the hour of
> Spring daylight saving transition was 0000 (midnight)  to 0100.  I would
> like to confirm these entries are in the tz database.

Yes, tzdb says the 10 years 1930/1934 and 1937/1941 all did that, as can
be seen from the extract Brian emailed.

> I have numerous
> references that daylight saving time was also observed in
> America/Regina  during world war ONE ( 1914-1918) and I am uncertain if
> those entries are in the tz database or the hour of those transitions.

As Brian mentioned, tzdb has these two transitions for 1918:

Sun Apr 14 08:59:59 1918 UT = Sun Apr 14 01:59:59 1918 MST
Sun Apr 14 09:00:00 1918 UT = Sun Apr 14 03:00:00 1918 MDT

Sun Oct 27 07:59:59 1918 UT = Sun Oct 27 01:59:59 1918 MDT
Sun Oct 27 08:00:00 1918 UT = Sun Oct 27 01:00:00 1918 MST

However, there's a good chance these data entries are incomplete for
Regina. It's also possible that the entries for 1930/1941 are wrong. For
details, please read the commentary for Regina in the northamerica file.
Here's the current development version:

https://github.com/eggert/tz/blob/master/northamerica