On 03/19/2018 03:09 PM, Michael H Deckers via tz wrote:
the switch was done at a single instant, when UT was 1883-11-18 + 18 h. This differs from the current description in tzdb which uses different instants for different target time zones -- not a likely choice for simplifying a nationwide railway schedule.
But we have a reliable eyewitness account that New York had two noons that day. See the quotation from William F. Allen in the "northamerica" file, taken from Bartky's 1989 paper <http://dx.doi.org/10.2307/3105430>. So the New York switch cannot have been at 18:00 GMT that day, as that would have meant two instances of 13:00, not two instances of 12:00. Bartky writes that it was called the "day of two noons" because eastern parts of the new zones observed noon twice, which wouldn't have happened if everyone switched at 18:00 GMT. Allen also wrote that New York got its time signal from the Naval Observatory, not from the Allegheny Observatory. Back then, different observatories were competing for the time-setting business. Perhaps the SpaceWatchtower sources included Allegheny partisans? (It's hard to tell from its source list.) It's possible that parts of the US switched at around 12:00 local time while other parts switched at 18:00 GMT; but if that's the case, I'd like sources for which parts switched which way.