[tz] Adding verified historic details

Guy Harris guy at alum.mit.edu
Wed Sep 4 22:46:33 UTC 2013

On Sep 4, 2013, at 2:57 PM, Lester Caine <lester at lsces.co.uk> wrote:

> Guy Harris wrote:
>> On Sep 4, 2013, at 2:15 PM, Lester Caine <lester at lsces.co.uk> wrote:
>>> If you want a 1970+ only database then it should be advertised as that,
>> As far as I know, he wants the ability for packagers of the database to create, from the tzdb, 1970+-only databases if they deem pre-1970 data not worth keeping, not to permanently expunge pre-1970 differences from the tzdb itself.
> Then it's up to the distributions to do that - in a manor that is compatible wth the rest of their infrastructure ... such as not having a pre-1970 calendar.

Well, it's up to them to do whatever they choose, including merging tzids that correspond to regions that differed only prior to 1970 *and* having calendar software that doesn't stop at 1970.  You may disapprove of that choice, but you should take that up with them.

>>> and we can create a second one which has all of time
>> If we're not going to be adding new tzids by splitting zones that differ only pre-1970, and there are currently tzids that cover a single region that has different pre-1970 behavior in different parts of the region, we're not going to have "a second one that has all of time" accurately.
> A proposal has been put forward to prevent the creation of new timezones just for different locations prior to standard time being adopted. Pre-standard time LMT is assumed and calculated independent of TZ.

What about locations that are in a single tzdb zone, so that they currently don't differ in post-standard-time, but that we discover didn't all maintain the same GMT offset subsequent to the adoption of standard time or didn't follow the same rules for "shifted time" (to avoid the term that implies that daylight is being saved and the term that implies that this shift only happens during the summer :-))?

And what does the adoption of standard time consist of?  Is "standard time" only specified by the actions of a government (whether national or sub-national or local), so that standard time can only be adopted by a government, or does it include some form of non-governmental standardized time (e.g., railway time) very broadly used?  For the former, we can (at least for national governments, and probably sub-national governments in many nations, and perhaps even local governments) establish a fairly firm date for the adoption; for the latter, that might be more difficult to establish, unless we simply decide to say that, once the railways agreed on standardized time, "OK, these regions are on standard time" and ignore people who weren't keeping railway time.

> If there is documentary evidence of a different time pattern such has been added for the Isle of Man and is about to appear for the channel islands then it should be allowed not blocked.

As, *for those locations*, that creates no new tzids (there were already tzids for the Isle of Man and, that's fine with me and, I suspect, Paul.

If, however, we were to discover documentary evidence that, for example, some towns in New Jersey chose not to adopt daylight savings time in 1918:


supporting *that* in the tzdb would require splitting America/New_York and creating a new tzid or tzids for those towns.

> So why are you dictating that pre 1970 data will be stripped?


I'm *not* dictating that pre-1970 data will be stripped from either the tzdb or from the version of the tzdb some supplier distributes, I'm just saying I won't get in the way of somebody stripping pre-1970 tzid differences from the copy of the tzid they distribute.

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