The character "u" is used to mark unknown parts of a date in the MARC 21 Format for Bibliographic Data. I'm not saying it has to be used in the proposed standard, but it's an example of where it has been used.
John Hostage Authorities and Database Integrity Librarian
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> -----Original Message-----
> From: Discussion of the Developing Date/Time Standards
> [mailto:[log in to unmask]] On Behalf Of Jakob Voss
> Sent: Monday, November 22, 2010 13:30
> To: [log in to unmask]
> Subject: Proposal to change unknown marker from 'u' to 'x'
> In the current draft "The character 'u', used to mean "unknown",
> replaces a single character and signifies that the value of that
> character is unknown." Is this based on an ad-hoc decision or on
> practical use cases? I have never seen anyone using the character 'u'
> mark unknown parts of a date. But I regularly see the use of character
> 'x' or 'X' for this purpose. So hereby I propose to change the
> 'u' to 'x'. In doubt, take a random set of people from the street and
> ask them which variant looks more familiar to denote a date that is not
> exactely known:
> Wikipedia already uses this notation, which should be argument enough,
> for instance:
> Another argument is the cultural depence of 'u' for 'unknown', which is
> bound to English language. In contrast 'x' for 'crossed out' is also
> known by many non-English speakers.
> The change does not influence other parts of the specification, so it
> should be no big issue.
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