[tz] Dealing with Pre-1970 Data

[Paul Eggert]

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.  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.  This value
is not deducible from what's in the tz database now,
unfortunately.