[tz] Dealing with Pre-1970 Data

[Guy Harris]

On Aug 30, 2013, at 1:41 AM, Lester Caine <lester at lsces.co.uk> wrote:

> David Patte ₯ wrote:
>> Though it has been handy to have some pre-1970 data within the tz database, I
>> don't see that it is the best solution for historical tz data in the long run.
>> It is clear that that going back far enough in time there is a different LMT for
>> every 15 seconds of latitude.
> 
> Only since 1884 ;) But even then as Paul as said, timezones where not actually agreed on, only where the base UTC time was defined. The 'local time' was still synchronised to the sun overhead at noon, hence there was no 'zone'

...and therefore entries prior to the establishment of timezones don't belong in the tzdb, given what the "z" in "tzdb" stands for.

If we're obliged to leave them in the tzdb for backwards compatibility purposes, we should:

	accompany them with a disclaimer that they're not actually meaningful, for all the reasons discussed here (not all locations within a time zone necessarily had the same LMT value before the establishment of the zone; different people *in the same city* might have had clocks different from each other by significant amounts; etc.), explicitly stating that supporting conversion of times prior to the establishment of a standard time zone in a locale is out of scope for the tzdb;

	freeze them and devote no effort to updating them;

	not create any new tzdb zones if the only reason for the new zone is "before standard time was established, these two locations had different LMT".

Historical rules *subsequent to* time zone establishment, however, are arguably worth keeping and perhaps even updating, albeit perhaps with a disclaimer saying we can't guarantee historical accuracy and/or that they are subject to change due to additional historical information being found.