Library of Congress has proposed an extended date/time definition for use with ISO 8601 and possibly with W3C as an XML schema type.
The problem: No standard date/time format meets the needs of XML metadata schemas. W3C XML Schema built-in types xs:date, xs:time, and xs:dateTime are inadequate, as is W3CDTF, and TEMPER. ISO 8601 and the W3C schema are incompatible. The LC proposal addresses that and adds BCE dates, open date ranges, and useful/necessary concepts like "uncertain" and "approximate" to the definition and the format.
The proposal could be incorporated into schemas such as MODS and METS. (Note: it is already in use within the PREMIS schema.) It may be proposed for standardization in ISO 8601 or it might be proposed to W3C for adoption as an XML schema type – the benefits of this are clear, among them: strict validation would be supported.