[F&O] IBM-FO-012: Return types are not valid SequenceTypes

(IBM-FO-012) Section 5.1, Constructor Functions for Built-in Types: The 
return types for the xs:dateTime, xs:time, and xs:date functions are not 
valid SequenceTypes. We do not have tuple-types in the data model. In 
general, handling of timezones in dates and times needs to be clarified 
throughout the Data Model and F&O books. All our functions should be 
closed under the Data Model and should not return things that are not Data 
Model values (like "tuples").

--Don Chamberlin