[fitswcs] FITS WCS Time Paper

[Rob Seaman]

On Mar 27, 2012, at 3:56 AM, Lucio Chiappetti wrote:

> - section 3.1 datetime "type"
> 
>   the datetime "type" defined here is an ASCII string representation,
>   so it applies MOSTLY to header keywords, which are not "strongly
>   typed" but "heuristically typed".
> 
>   One might possibly have cases in which a datetime string is stored
>   in an ASCII table, or binary table column, but the latter is definitely
>   less efficient than one of the floating point representations.
> 
>   So datetime is not properly a type (TFORM) but a format (TDISP).
>   One could, if not satisfied with the heuristic interpretation,
>   define a specific format descriptor (I remember I had a custom H
>   format in a plot labelling program to plot times as hh:mm hh:mm:ss or
>   hh:mm:ss.ff etc.) specifying things like digits for year and
>   fractional part of seconds.

FWIW, IRAF (CL and SPP) support this, e.g.:

	ecl> real time = 21.993
	ecl> printf ("%h\n", time)
	21:59:34.8

	ecl> real angle = 321.339
	ecl> printf ("%12.3H\n", angle)
	21:25:21.360

Where %H converts degrees to hours (divides by 15).  Also, unquoted sexagesimal is a real value (type or format as the spirit moves you):

	ecl> printf ("%7.3f\n", 21:59:34.8)
	 21.993
	ecl> = 21:59:34.8
	21.993
	ecl> = "21:59:34.8"
	21:59:34.8

> - section 3.1 datetime 5-digit year format
> 
>   *) the change from 4 CCYY to 5 signed digits requires a CHANGE to
>      the existing FITS standard (4.4.2.1)
> 
>   *) question: is the extension to 5 digits part of "true" ISO 8601 ?
>      I have no access to the expensive ISO standard, and the info on
>      the net is contradictory.
>      If it weren't, we should not call it ISO 8601 but ISO 8601 like.
> 
>   *) I presume the usage of 5-digit years will be limited to specific
>      sub-communities. What are their needs ? Should we limit to 5 digits
>      or can have an arbitrary number of digits between sign and first
>      dash?

Not too soon to get started:

	http://en.wikipedia.org/wiki/Year_10,000_problem

Some are already using 5 digit years:

	http://longnow.org/about/

That said, it would be unsurprising if using such dates caused current software to hiccup.

Rob