[tz] Dealing with Pre-1970 Data

[Guy Harris]

On Sep 2, 2013, at 1:09 AM, Paul Eggert <eggert at cs.ucla.edu> wrote:

> Guy Harris wrote:
>> If somebody happens to have a seconds-before-the-Epoch value from before 1847,
>> if they want to know what it corresponds to as year/month/day/hour/minute/second
>> in local time, they'd better say what the longitude was of the locality in question;
>> if they didn't, the correct answer is "mu"
> 
> Sure, but if we're talking Europe/London, the same is also true
> for timestamps from 1847 through 1880, because Europe/London stands
> for a region where clocks didn't agree until August 2, 1880.

Yup.

> So, if we want localtime() to return NULL when the answer is indeterminate,
> then we need to find the earliest time in each zone such that the
> zone's clocks have all agreed since then.

If we're not going to create zones for regions whose time offsets differed prior to 1970, then, yes, perhaps we *should* fail for times before then.

(Alternatively, we could pick a "most of the region is on standard time, and that's good enough" date and use that, e.g. perhaps 1855 for the UK:

	http://web.archive.org/web/20080828054933/http://www.greycat.org/papers/timetrav.html

.)