> In this case, you might also write your function to emit some 
> kind of error condition if the incoming string is not in the 
> right form.

You can always do <xsl:message terminate="yes">.

One thing I've done occasionally is to define a union type that allows
xs:date or **error** as its member types. That's a bit cumbersome though.
Really, XSLT needs a proper exception handling mechanism built in to the
language.

Michael Kay
http://www.saxonica.com/