DALI 1.1, time annotations

[Markus Demleitner]

Hi everyone,

On Wed, Feb 17, 2016 at 04:25:21PM +0000, Mark Taylor wrote:
> That's fine, but I would also like to be able to mark columns in a
> VOTable that contain timestamps in other formats, e.g.: MJD, JD,
> decimal year (maybe others ... Unix milliseconds???).
> 
> At the moment there is no way to mark up a VOTable column in such a
> way that you can tell this is what it contains.
> Units are some help, as are UCDs which can (when present)
> distinguish between an epoch and a duration.  But none of these
> can distinguish between a JD and an MJD, because they can't
> specify a zero point.
> 
> This seems to me like a prime use case for xtypes.  So I would
> suggest at least the following xtype values:
> 
>     xtype="mjd"
>     xtype="jd"
>     xtype="decYear" (?)
> 
> Perhaps experts from CDS can advise on what forms timestamp
> columns actually exist in in the wild.

I have all of them in my database, for starters, and I'd dearly love
to have some way to mark this stuff up in a way that clients actually
understand it[1].

And until DALI 1.1 I'd actually have agreed to using xtype like this.
But DALI 1.1 defines the "interval" type using xtypes, and given
that's already in use in SIAv2 (and the SODA draft) it's unlikely
that we can banish that ghost back into the bottle again even if we
could reach a consensus that's desirable (you'd have my vote,
though).

This means that anything that we may ever want to use as interval
boundaries cannot be marked up using xtype (unless we made xtype a
compound -- shudder).  And at least for mjd that is very definitely
the case (both SIAv2 and SODA already have cases).

My take on all this is: We need to stop heaping hacks upon hacks and
finally do a usable *and used* STC data model including a VOTable
serialisation for it.  If we had had the courage to go the proper way
through a parameter data model for interval, we wouldn't be  in
the fix we're in now.  So, let's not repeat history here.

And therefore I can only invite everyone to participate in
discussions like

http://mail.ivoa.net/pipermail/dm/2015-November/005268.html

and

http://mail.ivoa.net/pipermail/dm/2015-February/005103.html

-- and of course in the development of the up-and-coming source data
model (watch the DM list for news on this).

With apologies for my moaning (but the STC mess really is a huge
thorn in my side),

        Markus


[1] As its first author I mention in passing that the STC-in-VOTable
note http://ivoa.net/Documents/Notes/VOTableSTC/ contains such
annotations, but given the deficiencies of the underlying data model
it may be for the better that it hasn't been taken up.