## 1232RE: [ISO8601] Re: meaning of time-interval

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• Feb 2, 2005
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Harry Shipley

* -----Original Message-----
* From: Pete Forman [mailto:pete.forman@...]
* Sent: 01 February 2005 17:31
* To: ISO8601@yahoogroups.com
* Subject: Re: [ISO8601] Re: meaning of time-interval
*
*
* If someone was born on 2004-02-29 when is their first birthday? You
* might say 2005-03-01 or 2005-02-28. Does that mean that they have the
* same birthday as someone born on 2004-03-01 or 2004-02-28?

[Harry Shipley]
The birthday is one year later; which year length (tropical, leap, Gregorian
ordinary or sidereal to name but a few) you choose from is up to you
depending on what you wish to celebrate. The problem is compounded by the
fact that no-one knows how long a year is, particularly because it is a
varying quantity. Part of the confusion here is that leap years are being
added to some dates and ordinary years to others; little wonder that
different start dates end up on the same anniversary date!

*
* There is a general expectation of addition that subtraction of the
* addend from the sum gives the original.
*
* Arbitrary arithmetic is not be possible using any units. You cannot
* add anything to 1997-06-30T23:59:60Z except for 1Y6M and similar. A
* small number of seconds would be okay but a large interval could not
* extend more than a few months into the future because of
* unpredictability of leap seconds. Any other units would produce a
* result with 60 in the seconds.

[Harry Shipley]
I can't see the problem with arithmetic. A date/time, plus a time interval
which is unambiguously related back to SI seconds leads to a second
date/time (assuming we know what leap seconds are to be added, but anyone in
possession of a date/time can modify it appropriately). We know how to
write that date time. Just as if we add 17 and 25 we get 42, we don't get
"three" in the tens column and "twelve" in the units. Everyone understands
how to carry from one column to the next; so we carry days to months or
hours to days, etc.

*
* The only way to do correct interval arithmetic is to use something
* like TAI. But that is off topic for ISO 8601.

[Harry Shipley]
I think is on topic to emphasise that the second is the SI time unit. Every
other unit used must be related to the second, and not used on its own. A
month is not a fixed time interval, but the varying time between changes of
month identifier, which are better identified by that change and not the
specification of a time interval.
*
* IMHO the intent of time-intervals involving duration in ISO 8601 is
* that the start + duration or end - duration should (must?) be a valid
* date-time without having to apply any normalization rules.
*
*

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