Reading the new draft specification of EDTF, I reflected about the 
usefulness of x's in #204.

I will not focus on the risks for confusions between x's and u's or such 
misinterpretations, even if those risks are real.

In all the examples in #204, the x replaces the last digit of a year. Is 
such a limitation part of the EDTF specification? Or should we assume that 
x's may be placed on other digits anywhere in a date?

1) Assuming that x's are only allowed to replace the last digit of a year:

I wonder whether there is a need for such a construction. Indeed


is easily (and more legibly) rewritten as


Furthermore, in most of cases, such multiple dates do not coincide with 
"decades" (an undefined term within EDTF) and

{1958..1959, 196x, 1970}

is probably better rewritten as


which is both shorter and more legible.

In such a case, I suggest to remove x's from the EDTF specification.

2) Assuming that x's are allowed on other digits that the last digit of a 

I think that we should make clear that x's are allowed to replace any 
digit, anywhere in a date. We should also be aware that the BNF will be a 
bit more tricky in such a case.

Thus I suggest to define more precisely WHERE x's are allowed to occur - 
if we still want them.

Of course, in some cases, x's would make it possible to write a much 
shorter expression. For example, let's consider

{1xx0, 2000}

which would mean all years ending with a zero from 1000 inclusive to 2000 
inclusive and obviously is MUCH shorter than what it would be if we do not 
allow x's on other digits than the last one in a year. I wonder whether 
there are enough use cases for such constructions.

In some cases, x's are maybe useful. Let's consider


which means at twelve o'clock every day during june 2011. According to the 
BNF, such a construction is not unambigously allowed, though. I do not how 
many use cases there are for such constructions.

As a summary for this e-mail, I suggest to reflect about the usefulness of 
x's and (if we still want to have them) unambigously formulate where they 
may occur.