Looking again at this:

 From: Bruce D'Arcus
> 2004-01-01/
> E.g. an interval with an unspecified closing date.
> Perhaps, depending on how the other discussion goes, we could also
> allow something like:
> 2004-01-01/?

The use cases are 

(a) the end of an interval is "open".   
Not "unspecified", not "unknown", but "open" as in "till the end of time"

(b) the end of an interval is "unknown"

(c) the beginning of an interval is "unknown".

(There is no use case for the beginning to be "open". However that case is currently represented in the BNF, why, I don't remember, but I don't see that it can hurt.)

 I think we can generalize "unknown" to "unspecified".  But I want to be clear that by "unspecified" we don't mean "open", we mean something close to "unknown"

Anyway, So 


Could mean the interval "2004-01-01 till the end of time"

Representing "unknown" is trickier.  I don't think we want to use 'u', as in


because 'u' is used as a digit replacement and using it in this manner would be inconsistent.

Using '?' would not have that problem, but '?' is otherwise attached to a date and means "uncertain about that date", where what we want to say here is "I don't know", and that would be inconsistent with other use of '?'.  "I don't know" isn't said anywhere else.

So, I think there is a workable solution to "the end of time" but I'm not sure about unknown/unspecified.