[DALI] timestamp format

[Arnold Rots]

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. That, for instance, applies to creation
date-and-time of FITS files.
You can reference STC or the FITS WCS Time draft.

  - Arnold

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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
USA
http://hea-www.harvard.edu/~arots/
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On Tue, Nov 5, 2013 at 3:19 PM, Patrick Dowler <
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
>
> --
>
> 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)
>
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