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I think I've changed my view on this (in the past few hours), I've thought it through with hopefully more clarity than before.  

I DON'T think we need to justify the before/after feature, because I think it fits naturally with our syntax (as a few of you have hinted).

We represent list using comma and double-dot, enclosing the list with square or curly braces to mean "one of" or "all of", 
so
[1667,1668, 1670..1672]
means one of the years 1667, 1668, 1670, 1671, 1672

and
{1667,1668, 1670..1672}
means all of the years. 


More simply,

[1670..1672]
means one of the years  1670, 1671, 1672

and
{1670..1672}
means all three years.


It seems a natural extension that 

[..1672]
would mean some year preceeding (or equal to) 1672

and
{..1672}
all years preceeding or equal to 1672.

And that

[1672..]
means some year following (or equal to) 1672

and
{1672..}
All years including or following 1672


And I would go further to suggest that we replace "open" with double-dot, so 

20040101/..
means the interval beginning January 1 2004 with no end date


I have more to say about intervals but will save it for a later message.

--Ray