> quotes cause problems
That may occur especially if quotes are put inside XML attributes, for 
example. I guess that EDTF dates will often be used as values in XML 
attributes. It can work thanks to entities or the use of single quotes or 
other syntactic subtleties, but it is more difficult to parse and rework - 
for example with regexes in XSLT.

I have a suggestion!

As of today's EDTF specification, colons are always followed by a digit 
(except in URIs). In xs:QName colons are NEVER followed by a digit. We 
could use prefixed names (that is a subset of xs:QName) to encode URIs. In 
their syntax described at they always contain 
exactly ONE colon and this colon is NEVER followed by a digit.

> to determine that an expression is a URI
This would be a "word" enclosed between spaces, tabs or end-of-lines (etc. 
as defined according to XML 1.1) and containing (among other characters) 
ONE colon, and this colon would always be immediately followed by a 
non-digit character.

Within XML, the namespace would be declared more or less globally as a 
namespace node on any appropriate ancestor-or-self element. Likewise, 
namespaces may be declared in turtle when appropriate, etc.

For example:

<myElement xmlns:lca="" edtfDateTime="lca:sh2001008867.rdf" />


In turtle, an example could be:

@prefix dc: <> .

@prefix edtf: <> .

@prefix foo: <> .

@prefix lca: <> .

foo:baz dc:date "lca:sh2001008867.rdf"^^edtf:temporalExpression .

or similar, depending on how we decide to fine tune the EDTF 
specification. Details about turtle are available at