For what it's worth, so would I.  I think this answers my intial 
question, which is what sent the thread off on this tack.

John

Quoting Tim Finney <[log in to unmask]>:

> I would vote for that too.
> 
> Tim Finney
> 
> On Thu, 2008-01-31 at 08:31 -0800, Martin Holmes wrote:
> > Conal Tuohy wrote:
> > > <date>
> > >         <date when="1640" cert="low"/>
> > >         <date notBefore="1620" cert="medium"/>
> > >         <date notAfter="1660" cert="high"/>
> > > </date>
> > 
> > This seems an elegant and flexible approach to me. It allows
> refinements 
> > like this, too:
> > 
> > <date>
> >          <date when="1640" cert="low"/>
> >          <date notBefore="1620" notAfter="1660" cert="medium"/>
> > </date>
> > 
> > where the certainty value applies to the whole date range. I would
> vote 
> > for this being incorporated into the guidelines as good practice.
> > 
> > Cheers,
> > Martin
> > 
>