[DALI] timestamp format

[Patrick Dowler]

The DALI doc already has the text pertaining to additional metadata for 
time values (time scale and reference position). The simple issue here 
is that the string is not an ISO8601-compliant string as we are saying 
both "there is no Z" and "it is UTC" (by default as we allow for time 
scale metadata to be included separately).


The choices are:

- do not use the Z and refer to FITS and/or STC, in whch case the 
document should not mention ISO8601 at all, or at least very carefully)

- use the Z and refer directly to ISO8601, which Dave argued is the 
least surprising to the periphery and outside the IVOA but incompatible 
with current practice in astronomy

To Z or not to Z...

Pat

On 11/05/2013 01:49 PM, Arnold Rots wrote:
> Time metadata are defined in STC for the IVOA.
> There the ISO 8601 string is defined as in FITS.
> Time zone information is irrelevant in astronomical applications,
> so that part of the ISO standard does not apply.
> Instead, providing a timescale is crucial and so that information is
> contained in a separate metadata item.
> In situations where events on the geoid are tagged it is admissable
> to assume UTC as default.

Admissable according to which spec? I assume you mean STC.

 >That, for instance, applies to creation
> date-and-time of FITS files.
> You can reference STC or the FITS WCS Time draft.
>
>    - Arnold
>
> -------------------------------------------------------------------------------------------------------------
> 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 cfa.harvard.edu <mailto:arots at cfa.harvard.edu>
> USA http://hea-www.harvard.edu/~arots/
> --------------------------------------------------------------------------------------------------------------
>
>
>
> On Tue, Nov 5, 2013 at 3:19 PM, Patrick Dowler
> <patrick.dowler at nrc-cnrc.gc.ca <mailto:patrick.dowler at nrc-cnrc.gc.ca>>
> wrote:
>
>
>     Comments from the TCG review have pointed out either an
>     inconsistency or at least poor referencing in the PR-DALI-1.0 document.
>
>     ** The problem
>
>     The DALI document formalises the current restricted usage of ISO8601
>     timestamp strings within DAL services. Specifically, this common
>     usage (I think first written down in VOResource and coming from
>     FITS) specifies that dates are
>
>     YYYY-MM-DD
>
>     and that date+time are written as
>
>     YYYY-MM-DDThh:mm:ss[.sss]
>
>     where the T is the literal separator between date and time. This
>     would be called a "profile" of ISO8601 since it specifies a subset
>     of the standard. However, IVOA practice, which DALI kept, is to
>     always require such timestamps to be UTC (detailed language is
>     slightly more specific), while ISO8601 explicitly states that the
>     above is interpretted as "local time" and a trailing Z is required
>     if it is UTC.
>
>     So, common IVOA usage (and as I understand the "ISO8601 profile" in
>     the FITS community) is not consistent with ISO8601.
>
>     ** What to do?
>
>     Well, we cannot just say this is a valid "ISO8601 profile" since
>     stock software that groks ISO8601 will *not* interpret our timestamp
>     strings correctly.
>
>     1. Fix the inconsistency with ISO8601 by requiring the Z for UTC.
>
>     2. Stick with the current style and
>
>     --2a. reference FITS usage instead of ISO8601 and *do not* state
>     this is an ISO8601 profile, just a similar syntax
>
>     --2b. claim that in the IVOA "local" means the same as UTC
>     (admittedly this is kind of sketchy)
>
>     --2c. is there a better justification than backwards compatibilty?
>
>
>     If someone can provide the correct FITS reference, that would help
>     with 2a.
>
>     Here is a good overview of ISO8601 without having to buy it:
>
>     http://www.cl.cam.ac.uk/~__mgk25/iso-time.html
>     <http://www.cl.cam.ac.uk/~mgk25/iso-time.html>
>
>     --
>
>     Patrick Dowler
>     Canadian Astronomy Data Centre
>     National Research Council Canada
>     5071 West Saanich Road
>     Victoria, BC V9A 2L9
>
>     250-363-0044 <tel:250-363-0044> (office) 250-363-0045
>     <tel:250-363-0045> (fax)
>
>

-- 

Patrick Dowler
Canadian Astronomy Data Centre
National Research Council Canada
5071 West Saanich Road
Victoria, BC V9A 2L9

250-363-0044 (office) 250-363-0045 (fax)