Subject: Re: DATE-OBS='31/12/99'

From: Pierre Maxted <pflm at star.maps.susx.ac.uk>
Date: 28 Jun 1996 18:54:48 GMT

-- Preben Grosbol <pgrosbol at eso.org> wrote:
: : :
> It is very important not to fool old FITS readers.  Thus, the first
>8 characters of data value strings for 'DATExxxx' keywords must be
>maintained as 'dd/mm/yy'.  I agree with Rob that the double keyword
>solutions e.g. 'CENTxxxx' have a number of disadvantages.  Also a whole
>set of new e.g. ISO8601 formated data keywords may be too much.
: : : 
If our first priority is indeed to "not to fool old FITS readers" then 
there is _no_ way to cope with DATE-OBS='31/12/99' because a two-figure
year is inherently ambiguous once somebody starts writing data after  
31/12/99 now that pre 1901 data has been archived as FITS. In this case
the best you can do is to make sure no FITS reader can ever read in a year
and then interpret it incorrectly. A solution I'm sure many people won't
like is to strictly reserve the two-figure year format for the years 1900
to 1999, any other date being written 'DD/YY/XX' e.g. '01/01/XX'. You
might call this the "fail-safe"  solution since it provokes an error
whenever there is some ambiguity over the date. I leave the argument over
the required new keyword containing a four-figure year to others.
 
- Pierre

  _-_-_                                             _-_-_
 |     Dr Pierre Maxted (pflm at star.maps.susx.ac.uk)      |
| |       Astronomy Centre, University of Sussex        | |
 |            Falmer, Brighton, BN1 9QH                  |
  -_-_-         Procrastinate now!!!                -_-_-