On 05/04/2015 01:48, Dan Asimov wrote:
On Apr 3, 2015, at 7:27 PM, Keith F. Lynch <kfl@KeithLynch.net> wrote: ... Twenty-five hours. The default International Date Line is exactly opposite the prime meridian, hence is in the middle of a time zone, not on the border between two of them. So some clocks are 12 hours ahead of Greenwich when others are 12 hours behind.
Nope.
Per Keith's observation, at any given instant the nominal times at different points on earth (in the Idealized Dan Asimov Model) are t-24h, t-23h, ..., t for some t. So, at some instant it first becomes midnight-plus-epsilon on day D somewhere. In terms of the previous paragraph, this must be time t. Exactly 24 hours later, the nominal times are t, ..., t+24h, so now in the "slowest" time zone it has just become midnight plus epsilon on day D. All the other time zones are ahead of that. Just under 24 hours later than that, in that time zone it is just about to be the end of day D. All the other time zones are ahead by at least an hour, so in those time zones it is no longer day D. Thus, 48 hours elapse between the first and last instants at which it's day D somewhere on earth. -- g