ISO 8601 and STC

[Arnold Rots]

These issues were considered at length in the FITS community in the
late 90s.  See:

	http://www.cv.nrao.edu/fits/documents/standards/year2000.txt

The context was that a change in date formats was needed by 2000.  The
decision was to use a subset of of 8601.  Basically, I am using the
same framework in STC in the definition of the ISO8601 format of
astronTimeType, including the recommendations from the appendix.
Note that we allow the seconds to go to 60.
There is no real concept of time scale in 8601 (though 'Z' is allowed
for UTC, but this was on purpose not included in the FITS subset;
actually, 8601 is more concerned with time zones), hence the required
presence of a time scale.

It would probably be good to say explicitly that pre-1972 UTC times
will be interpreted as UT.

For BCE times ISO 8601 will not suffice, but I supect that JD times
are more useful in that case anyway.

There is one time issue left that I am considering while working on a
new version of the STC Working Draft:
JD (or MJD, for that matter) is cast as a decimal which allows
arbitrary precision.
That is exactly what we need, but it requires that clients are aware
that they can't just stuff the value in a float or a double.
The alternative is to follow the time-honored tradition of making JD an
array of two doubles where the "true" JD value is the sum of the two.
Any comments?

  - Arnold

Norman Gray wrote:
> 
> Greetings,
> 
> On 2004 Nov 30 , at 19.13, Norman Gray wrote:
> 
> > So the fact you can't specify a timescale, and it can't (depending on 
> > how fussy you're being) specify leap seconds, make it not ideal for 
> > astronomical use.
> 
> ...and I meant to mention that it's also formally undefined for dates 
> prior to the definition of UTC.
> 
> This means that 8601 could be almost completely adequate for astronomy, 
> if it were slightly augmented by formally permitting the seconds 
> counter to run to 60, and specifying a timescale for dates before 1986 
> (GMT?).
> 
> Norman
> 
> 
> -- 
> ----------------------------------------------------------------------
> Norman Gray  :  Physics & Astronomy, Glasgow University, UK
> http://www.astro.gla.ac.uk/users/norman/  :  www.starlink.ac.uk
> 
--------------------------------------------------------------------------
Arnold H. Rots                                Chandra X-ray Science Center
Smithsonian Astrophysical Observatory                tel:  +1 617 496 7701
60 Garden Street, MS 67                              fax:  +1 617 495 7356
Cambridge, MA 02138                             arots at head.cfa.harvard.edu
USA                                     http://hea-www.harvard.edu/~arots/
--------------------------------------------------------------------------