[tz] Issues with pre-1970 information in TZDB

[Guy Harris]

On Sep 23, 2021, at 3:48 PM, Tom Lane <tgl at sss.pgh.pa.us> wrote:

> Guy Harris <gharris at sonic.net> writes:
>> On Sep 23, 2021, at 3:20 PM, Tom Lane <tgl at sss.pgh.pa.us> wrote:
>>> For example, if Norway someday changes its laws such that
>>> it's no longer identical to Europe/Berlin, then it's a whole lot less
>>> painful all around if Europe/Oslo already exists and is already being used
>>> by most of the affected users.  They'll just automatically get the right
>>> time from a tzdb update, without having to adjust their settings.
> 
>> It already exists, *and will continue to exist with Paul's most recent changes*.
> 
> Perhaps I should have used a different example.

The only such example would be one of the tzdb regions that has already been removed, rather than moved to backzone; zones that *were* moved to backzone continue to exist as aliases, and the data is available.

> Or are you just spreading confusion for the sake of it?

No.

I'm pointing out that the data doesn't *disappear*, nor does the ability for software to use Europe/Oslo or America/Montreal or Africa/Asmara as a tzid.

However, software that uses tzids might limit itself to tzids corresponding to Zones rather than to Links, even in cases where the tzid does have a Zone in backzone.  If so, then the automatic adjustment you mention won't happen; for software that continues to use tzids corresponding to Zones in backzone, it will.

So the question is "what software limits itself to tzids corresponding to Zones rather than Links?", and I suspect the answer is "enough software that moving Zones to backzone could be disruptive".

Perhaps if there were three categories of tzids:

	tzids corresponding to Zones in the main files;

	tzids corresponding to Zones in backzone;

	tzids corresponding to Links regardless of whether you use backzone or not;

(which might involve removing backzone in favor of some other mechanism for putting tzids into those categories) that might make it more likely that tzids corresponding to Zones in backzone will be used by software.

That doesn't solve the pre-1970 data issue, but, again, we need to decide what we want to do with that.