Standardising units and formats (and ref frames?) in transmission

[Alberto Micol]

> On 19 May 2009, at 21:16, Rob Seaman wrote:
>
> On May 18, 2009, at 12:08 PM, Arnold Rots wrote:
>
> I'm afraid we have to accommodate time in ISO-8601 as well as MJD (or 
> JD); both have specific application. Besides, there are times that 
> cannot be expressed in ISO-8601 (since it has limited range).
>
> A couple of points here. As Arnold says, even if a single time 
> representation were possible, it could not be ISO-8601. Similarly, MJD 
> has an initial horizon of 1858 and even JD runs negative before the 
> 48th century BC.

Excuse my ignorance, but why is ISO8601 not a possible candidate?
According to wikipedia (http://en.wikipedia.org/wiki/ISO_8601):

> As a point of interest, ISO 8601 fixes a reference calendar date to 
> the Gregorian calendar of 1875-05-20 as the date the Convention du 
> Metre (Metre Convention) was signed in Paris. However, ISO calendar 
> dates before the Convention are still compatible with the Gregorian 
> calendar all the way back to the official introduction of the 
> Gregorian calendar on 1582-10-15. Earlier dates, in the proleptic 
> Gregorian calendar, may be used by mutual agreement of the partners 
> exchanging information.

and also:

> To represent years before 0000 or after 9999, the standard also 
> permits the expansion of the year representation [+/-YYYYY], but only 
> by prior agreement between the sender and the receiver. An expanded 
> year representation must have an agreed upon number of extra year 
> digits beyond the four-digit minimum and is always prefixed with a + 
> or − sign with the convention that year 0 is positive.

I thought the IVOA could build on that, exploiting the "mutual 
agreement" possibility therin indicated.

Am I missing something?
Is it not enough for the VO to agree on those (already indicated) 
extensions?
Does the VO need to interoperate with entities outside the VO?
If so, can't we make an agreement with those?

Thanks,
Alberto