The precise proposed addition to the text of ISO/IEC 29500-4 is:
§10.7, "Additional representation for dates and times (Part 1, Section
18.17.4 )"
For a document of a transitional conformance class, each unique instant in SpreadsheetML time shall be stored
as an ISO 8601-formatted string or as a serial value.
This would override, for files of the "transitional" conformance type,
the statements in Section 18.17.4 which allow only the ISO 8601 date
format.