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DATETIME  November 2015

DATETIME November 2015

Subject:

Re: 1/27 conference call notes: an instant can't be on 3 different days

From:

"Edward C. Zimmermann" <[log in to unmask]>

Reply-To:

Discussion of the Developing Date/Time Standards <[log in to unmask]>

Date:

Tue, 24 Nov 2015 17:45:17 +0100

Content-Type:

text/plain

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Parts/Attachments

text/plain (93 lines)

Correct.

Munich is 12 hours ahead of Pago Pago.. Christmas Island is UTC+14 or 13 
hours ahead of Munich. Baker Island is UTC-12. It is 13 hours behind 
Munich.

This brings up an interesting issue. Imagine a date/time reported in 
minutes but without time zone. Here we have a readability of perhaps minute 
but an accuracy still of under +- 1560 minutes (roughly +- 1 day) to 
resolve to some other location.

This is a relevant search problem.

Pretend we have the specification of the time zone but not the time in a 
database. A match of a date with a time specification and time zone can be 
done under +-1 day. Example.. Say I have a report of an event in London 
(GMT) on 24 Nov 2015. This can match a search for 24 Nov 2015 or 25 Nov 
2015 in Munich (day precision) but the specification of time, all of 24 Nov 
but only up to 0:59 AM in Munich on the 25th (a total span of 25 hours!). 





On Tue, 24 Nov 2015 16:31:44 +0100, [UTF-8?]SaaÅ¡ha Metsärantala wrote
> Hello!
> 
> > an instant can't occur on three days. [...]
> 
> > I think that it is always the case that there are no more than two 
dates in use at any one time. [...]
> 
> > for practical purposes, every instantaneous event occurs on exactly two 
different days. Am I thinking clearly?
> Not really! Let's keep in mind that time zones span between -12:00 to 
+14:00. That makes room for three calendar days! Let's try to print the 
number of seconds since 1970-01-01T00:00:00Z (UTC) "the epoch" in a POSIX-
compatible CLI terminal, thus writing:
> 
> date -d '2015-11-22T23:00:00-12:00' +%s
> 
> date -d '2015-11-23T12:00:00+01:00' +%s
> 
> date -d '2015-11-24T01:00:00+14:00' +%s
> 
> each of these will give the very same answer, namely
> 
> 1448276400
> 
> and that is the same moment in time, despite of the fact that there are 
three different "wall calendar" dates involved.
> 
> > the geographical location that you were measuring that time at, and 
that information tends to be abstracted into timezone offsets.
> The geographical location is related to timezones, but geographical 
locations are different from timezone offsets.
> 
> > > refer to the period of time *between* the first instant and the last 
instant of year we call "1980", inclusive.
> > [...] the interval notation would be used.....
> 
> > '1-1-1980/12-31-1980'
> 
> > ..... is as close as you will get with the edtf spec: the interval 
beginning some time during 1-1-1980 and ending some time during 12/31/1980.
> Likewise
> 
> 1980-01-01T00:00:00Z/1980-12-31T24:00:00Z
> 
> would be the duration between (1) some instant during the first second of 
the year 1980 and (2) the very last instant of that year. In other words, 
it would be between 31622399 and 31622400 seconds long, depending om "when" 
(during that second) within 1980-01-01T00:00:00 the measure was made. There 
were no leap seconds that year.
> 
> Of course, we could write
> 
> 1979-12-31T24:00:00Z/1980-12-31T24:00:00Z
> 
> to get an exactly 31622400 seconds long period. Kepp in mind that 1979-
12-31T24:00:00Z is the very last durationless instant at the end of 1979, 
which in this case comes AFTER the 1979-year's leap second (23:59:60 on 
1979-12-31).
> 
> Regards!
> 
> [UTF-8?]Saašha,


--

Edward C. Zimmermann, NONMONOTONIC LAB

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