## 1236Re: [ISO8601] Re: meaning of time-interval

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• Feb 5, 2005
In a message dated Mon, 31 Jan 2005 13:33:25 EST  hjw writes
Many reports require the month-end date.  It is easy to do, just advance to the first day of the next month and subtract one day.
Agree. Recently I found the following solution for this.

Days referred to the begin of month are numbered from 01 to 49. The days 01 to 28  have the usual meaning and must not be normalized,
others can be normalized easily, because they are continuously counted into the next month.

Days referred to the end of month are numbered from 50 to 99. The day 99 designates always the last day of month, 98 the day before, and so on.
In a specific month this can be normalized easily as you mentioned.

Actually I used this method to address yearly events like "last Friday in February". A calendar time like 2005-02-93T01:00+02:00 can be converted easily to zulu-time (2005-02-92T23:00Z) within this system.

In a message dated  Tue, 01 Feb 2005 16:31:28 +0000  Pete Forman writes
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?
Alternatively the yearly birthday could be defined as the last day of February -- or the 60th day of the year. This way the birthdays over some years will be different.

There is a general expectation of addition that subtraction of the
addend from the sum gives the original.
Agree. And the expectation is met if any quantity of calendar minutes is added on a calendar time like 2005-01-02T20:00. If SI-minutes (i.e. a multiple quantity of 60 SI-seconds) are added, information about leap seconds is needed to determine the final calendar time.

In a message dated  Tue, 1 Feb 2005 09:02:37 -0800 (PST)   John Steele writes
But certain combinations using duration DO result in invalid start or end times. Do we just not speak of them (you aren't allowed to use "duration" in those cases) or do we have normalization rules? I think normalization rules make more sense, but some cases, either of two values may seem "reasonable." They can either be agreed by the partners or standardized within the scope of the standard. I think it would be better to have proposed normalization rules in the standard "unless otherwise agreed by the partners."
As far as I understand (now), ISO only provides SI-units for the definition of a duration (1M=60S,1H=60M,1D=24H). And if there is no information about leap seconds the notation <start>/<duration> cannot be safely converted to <start>/<end>.

In a message dated  Tue, 01 Feb 2005 19:27:43 -0500   NGUYEN Adam writes
I wonder what would happen should I set the alarm to go off on every 31st day, or a
similar day number within a month. Perhaps it will sound only seven times a
year (on the months with 31 days)? If not, I would guess the next logical
thing it should do would be to sound on the last day of every month.

Any opinions?
An option would be nice to switch the reference from begin of month to end of month.
PS: I wonder why ISO 8601 doesn't allow for time zone offsets beyond
minute-resolution (eg. YYYY-MM-DDTHH:mm:ss±hh:mm:ss.sss…) (like that
historical Netherlands offset having centisecond-resolution).
IMHO this rule is very reasonable referring leap seconds, which would otherwise cause much more headache.

In a message dated  Wed, 02 Feb 2005 13:34:24 -0000  piebaldconsult writes
But the presence of leap seconds means that not even days, hours, and
minutes are fixed time units, so the portion of the standard you
quoted is in error.
Probably you mean calendar days, calendar hours and calendar minutes, but it was about
SI-days, SI-hours and SI-minutes which have fixed conversion factors to SI-second (24,60,60).

In a message dated   Wed, 2 Feb 2005 17:05:38 +0100  Harry Shipley writes
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!

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.

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.

Agree with one additional remark: Besides the SI time units as defined by ISO 8601, the calendrical time intervals are useful for some applications.
E.g. 2005-02 + 14 calendar months means 2006-04.

In a message dated   Fri, 04 Feb 2005 04:51:35 -0000  John Hynes writes
Of course, few now count days this way, and even the Church has
recently started following an ordinal count.  And you can celebrate
your birthday any day you wish; my brother celebrates his in June,
since he was born on Christmas Eve, and that's a sucky time to
celebrate a birthday.
Good idea.

In a message dated   Wed, 02 Feb 2005 15:08:27 -0800   Tex Texin writes
The question is further complicated by the fact that you didn't specify
the timezone the person was born in. Ignoring leap days, if someone is
born on 2002-02-02 in time zone -1200
and someone else is born on 2002-02-02 in time zone +1200, were they
born on the same day or not?
How about leap hours (aka transitition times from DST to Standard)?
Babies might be mistaken if they are born in the same location at the same local time.

In a message dated  Fri, 04 Feb 2005 21:30:57 -0500  NGUYEN Adam writes
There's a +1300 (+1200 in DST) too. ;-) That timezone problem
could be solved by converting both times to UTC.
UTC/GMT +14:00 (Christmas Islands).  from  http://www.worldtimezone.com/utc/utc+1200.html
Not sure if there is any DST.
About Chinese (?; I thought Korean was like this), the question of
how old someone is can be ambiguous in that respect, if you take that into
consideration. Would asking someone's date of birth be as ambiguous?
Wouldn't it be more safe to ask for the year of birth, or even month, day, etc., than to ask how old is someone?

Probably another aspect yet not covered in this thread:
A time interval like 2005/2006 may have a particular meaning, e.g. 12 calendar months, but considered as a representation with reduced precision the duration is uncertain and might be anything between 0 and 24 calendar months.