Dates

[P.Hill & E.Goodall]

Andy Waddington on Cave Surveying wrote:
> On Wednesday 2003-12-10 17:28, Olly Betts typed:
> 
>>What I've seen so far suggests that most software opts for the day,
>>month, and year (and so implicitly is limited to 1AD onwards).  

Variations or extensions of ISO-8601 others have suggested
include labeling a date with additional tags like the calendar used (Julian, 
Gregorian, or even Islamic or with AD BC modifiers), but ISO-8601 suggests
the use of a pure "proleptic Gregorian Calendar" (no use of BC and no
shifts to Julian etc.)

> Does ISO 8601 say anything about such dates, or is it purely an international
> standard for the representation of contemporary dates ?

It is intended for something a little broader than just current dates, but
I think we can say it is limited to historical dates representable in the 
present three part year, month and day of month scheme, so some
calendars don't fit the scheme.

I note the following from the spec.

"By mutual agreement of the partners in information
interchange it is permitted to expand the component identifying
the calendar year, which is otherwise limited to at most four
digits. This enables reference to dates and times in
calendar years outside the range supported by complete
representations, i.e. before the start of the year [0000] or
after the end of the year [9999].

When expanded representations are used, provisions should be made to prevent 
confusion of the expanded representations, with other date and time 
representations used by the application."
   --section "4.6 Expansion"

"This International Standard allows the identification of calendar years by 
their year number for years both before
and after the introduction of the Gregorian calendar. For the determination of 
calendar years and year numbers
only the rules mentioned above are used. For the purposes of this International 
Standard these rules are referred
to as the Gregorian calendar. The use of this calendar for dates preceding the 
introduction of the Gregorian
calendar (i.e. before 1582) should only be done by agreement of the partners in 
information interchange."
    -- section 4.3.2.1 (yes that is really the section number!)

On the point of carbon-14 dates.  Last I checked there weren't
many such dates that included months and day of months, so I think
such a consideration is outside the realm of ISO-8601.
But ISO-8601 is very flexible and all fields are optional as long
as you note somewhere what your variation or subset of 8601 is expecting.
An XML example might look like:
<artifact name="Pharaonic Burial Mask" ...>
   <dated method="C14" date="-2540"  delta="Y30" measuredOn="2003-12-12" ...>
   <dated method="OxfordEgyptianTimeline" date="-2525-07-04" delta="M15" ...>
...
</artifact>

I believe both the date, delta and measuredOn attributes are validluy
definable in terms of 8601 (including the use of delta times with Y or M 
modifiers) particularly if your format definition includes appropriate notes 
about how it conforms to "reduced precision", "trancated" or "expanded" 
representations.  There are enough such "if by agreement," when something other 
than the full format is used in the document to allow plenty of wiggle room.

> Although how the hell anyone is going to adopt a standard if it is only 
> available (at ridiculous expense) on paper beats me :-(

It is a curious world the ISO runs in which the documents that they
hold the copyright to are sold to raise money for standards they
expect everyone to use.

-Paul

p.s. If you need an electronic copy of the ISO-8601 and haven't found
an appropriate website, let me know. Such things are great sleep aids!