[tz] Dealing with Pre-1970 Data

[Paul Eggert]

Zefram wrote:
> This can only be done case-by-case, by examining the extensive commentary
> in the source files.

... and for pre-1970 data most of the commentary will
boil down to "see Shanks", which has been demonstrated
to be unreliable when we've checked it.  There is little
reason to trust that data.  Shanks gives no sources --
zero -- for any of his data.

As a consequence, not only are the LMT offsets unreliable,
for most current entries the transition dates from LMT to
standard time are unreliable.  Even if we were to create
an extended file that distinguishes between zones based
on pre-1970 data, we shouldn't create an extended-file
zone merely because it transitions away from LMT at a
differing date from an existing zone, because those
transition dates are typically no more reliable than
the LMT offset is.