If we consider only local time, when the sun is directly over the the
180degW (or 180degE) line of longitude the the western hemisphere will
have one date and the eastern hemisphere will have the previous date. By
the time the sun is directly overhead on the prime meridian the whole of
Earth will be experiencing the same day.
When we add time zones into the mix, and the fact that the international
date line zig-zags, it gets a bit more complicated, but I think that it
is always the case that there are no more than two dates in use at any
On 23/11/2015 9:57 PM, Edward C. Zimmermann wrote:
> Correct. But since the earthly instant might have occured anywhere on the
> planet we have a span of 3 days (+- 1). While any event measured in minutes
> can have occured on up to two different local time zone days if we don't
> have the measure of time nor even the location, we don't know if the event
> could have be reported by another location on the planet on the previous
> calendar day or the next.
> Imagine a date/time reported in a database as 2010-12-12T01:00 without
> knowledge of the "local time" where reported.. If it was Baker Island.. If
> it was Kiritimati.... Searching with a date/time specified in UTC in such a
> database.. What is the match? What is the precision?
> On Mon, 23 Nov 2015 20:35:54 +0000, Byrd, Donald A. wrote
>> There's only one International Date Line; time zones on the earth span
> just 24 hours. So it seems to me an instant can't be on more than two
> different days, regardless of where and when it occurs. Munich's time zone
> is nine (I think) hours west of the Date Line, so an event at, say, 11:45
> PM on Nov. 30 there is on December 1 anywhere more than a few hundred
> kilometers (or some such) to its east, up to the date line. It's 15 hours
> from the Date Line in the other direction, so the earliest it can be in any
> time zone is 15 hours earlier, i.e., 8:45 AM on Nov. 30.
>> Furthermore, an instant also can't be on _less_ than two different days,
> regardless of where and when it occurs. Well, it could be argued that an
> instant at exactly midnight somewhere is an exception! Anyway, for
> practical purposes, every instantaneous event occurs on exactly two
> different days.
>> Am I thinking clearly?
>> On Nov 19, 2015, at 5:13 PM, "Denenberg, Ray" <[log in to unmask]> wrote:
>>> So Neil [WINDOWS-1252?]Armstrong’s moon event is nominally 1969-07-20,
> [WINDOWS-1252?]that’s the [WINDOWS-1252?]“official” date that it occurred
> (nevermind that it was on the moon, [WINDOWS-1252?]let’s just say July 20
> is the [WINDOWS-1252?]“real” date, for discussion sake). Some observers may
> have recorded it as July 19. And I suppose somewhere else it was recorded
> as July 21. Would you call this [WINDOWS-1252?]“two-day” precision?
> Three-day precision?
>>> An event in Munich may occur close to midnight so it is the next day in
> Samoa (and the previous day somewhere else?) But it might be November 30
> in Munich, so it is December in Samoa, or it might be December 31 in Munich
> so it is the next year in Samoa. So the first case is 2 (or 3) day
> precision, the second is month precision and the third year precision. And
> maybe it occurred 2009, so there you have decade precision.
>>> Still, all these examples are based on cases where the estimate is no
> more than a day off. I [WINDOWS-1252?]don’t think I can convince the ISO
> people based on that. Are there other examples?
>>>> -----Original Message-----
>>>> From: Discussion of the Developing Date/Time Standards
>>>> [---- SNIP ----]
>> Donald Byrd
>> Woodrow Wilson Indiana Teaching Fellow
>> Adjunct Associate Professor of Informatics
>> Visiting Scientist, Research Technologies
>> Indiana University Bloomington
> Edward C. Zimmermann, NONMONOTONIC LAB