[LEAPSECS] BBC radio Crowd Science

[Zefram]

Brooks Harris wrote:
>It seems to me Tz Database does a great job of describing the meanings of
>local time after 1972,

The scope of the tz database is yet another thing that doesn't start
in 1972.  Though it's close.  The project actually aims to fully
cover 1970 onwards, and decisions about whether zones need to be
separately represented are fairly strictly based on that threshold.
(It has extensive pre-1970 data for the zones that it represents,
but makes no attempts to represent all zones in that era.)  The 1970
threshold is based on the availability of historical records, but that's
a fuzzy boundary.  The exact date of the threshold is an arbitrary choice,
probably influenced by the Unix epoch.

>Describing local time before 1972 is going to be a challenge. When was the
>Gettysburg Address delivered? Exactly how was local time understood at the
>time and how would it relate to modern timekeeping practice?

In that era there were timezones, but far more of them than there
are today.  The 1860s was a time of transition.  The older tendency,
being phased out, was for each town to have its own timezone, amounting
to local mean time of some reference location.  Differences between
towns' timezones were well known, but there was no attempt to make these
differences round numbers.  But the advance of railways was making the
differences troublesome.  Large railway systems came to operate on a
single time, amounting to a timezone of much greater geographical span,
and "railway time" gradually displaced per-town local time in civil use.

The US railways didn't adopt a single timezone system until 1883.
The Gettysburg Address predates that, so contemporaneous expressions of
time are probably based on Gettysburg mean time.  Unfortunately it was
(and remains) uncommon to explicitly note which time scale one is using
in civil affairs.  In general historical documentation on this sort of
thing is patchy.  I don't know what documentation is available regarding
the time of the Gettysburg Address specifically.

This is all quite well understood.  It's also off-topic for this mailing
list.  Any further enquiries regarding 19th century civil time should
go to the tz mailing list.

-zefram