UWS times

[Markus Demleitner]

Dear GWS,

Yesterday, Paul made a commit to the UWS document with the following
log message[1]:

> make the example time actually have UTC timezone, but do not mandate
> in the language - any timezone should be supported

-- and that,... well, shock is probably to hard a word, but you get
the idea.  I frankly have never noticed the intention of allowing
time zones in UWS time stamps, although admittedly the example Paul
fixed in this commit was clear enough.

So, excuse me for bringing this up so late, but I think this is
serious enough to even make me propose an erratum if we can't fix the
text any more.

This being machine readable data, I'd argue strongly we should
require, in effect, UTC (more exactly, timestamps without timezone,
since I'm not really concerned if someone uses TAI or TDB if they
like; there just shouldn't be time zones).

The main reason is: Supporting time zones is a *huge* implementation
concern (unless you put serious restrictions over ISO 8601), and
given things like daylight savings time, makes the standard
susceptible to lawmaking in the greater world.  Also, while explicit
time zone notation (e.g., CEST vs. CET) mitigates that particular
problem, local times have hours that pass twice (next autumn for us)
and ones that don't pass at all (this Sunday morning in much of
Europe).

Such considerations made us restrict timestamps in the VO to what
DALI 1.0 says in 3.1.2 (cf.
http://ivoa.net/documents/DALI/20131129/REC-DALI-1.0-20131129.html),
which is (after a correction that comes in in 1.1),
YYYY-MM-DD[Thh:mm:ss[.sss]].  So, the second reason I'd strongly
argue UWS should make clear what kind of times in which form should
be used is consistency with the rest of the VO.

I think the simplest way to make that happen would be by replacing
all references to [std:iso8601] to DALI and specify, where
appropriate, that the full form with T and time is to be used.

True, that is a change that might theoretically invalidate existing
services, but really: Does anyone put in anything else but UTC into
the time fields?

Cheers,

          Markus

[1] In case you're wondering: I noticed that because I subscribed to
the mailing list that sends around the volute commit messages.  You
can do so, too:

http://lists.g-vo.org/cgi-bin/mailman/listinfo/volutecommits