From: Saašha Metsärantala
> Maybe we should clarify that
> (A B) | C
> means the same that
> A B | C
> even without line break, doesn't it?

No, not unless we add to the rules section a rule for evaluation of "|" over tokens. As it is, 

A B | C

is ambiguous, that is, it isn't clear whether it means

(A B) | C


A (B|C)

> Several productions exclude negative years. Maybe, that is what we want.
> I suggest to clarify this point.

It is true that for some of the definition, for example, where 'u' is a placeholder, I haven't bothered to allow negative years. I don't see the need for that level of complexity. If someone has a concrete use case, I'll add it. 

> Furthermore, I can not find the production with
> > dateWithInternalUncertaintly

Right, I've added

dateWithInternalUncertainty =
| internalUnspecified

> A problem in today's BNF is that it allows dates such as
> 2011-02-31

We have ensured that months and days match up for basic dates.  To ensure this for the whacky dates is going to impose extreme complexity and I don't see why it's needed.