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In 10.3.1.3: >However, if the value is zero (0) months, the canonical form is "P0M". It should be noted that Schema does not prescribe a canonical form for P0M since the derivation does not produce one; this canonical form is defined explicitly by F&O. Dave Peterson for the XML Schema WG
Similar comment applies to canonical form for xdt:dayTimeDuration Dave Peterson for XML Schema WG
Duly noted! Thank you! Ashok Malhotra