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

[Rob Seaman]

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.  MJD will be acceptable for many purposes, but  
the VO also hopes to support theoretical data or extrapolations far  
into the past or future.

>  I believe that, as far as units are concerned, both 's' and 'd'  
> have a role to play.

And likely years as well.

Another issue - without digging too deeply into the leap second morass  
- is that conversion between overtly similar units may be difficult or  
fundamentally impossible.  A day is not equivalent to 86,400 seconds.   
One is defined by reference to SI, the other by reference to Mother  
Earth.

Rob

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