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In section 13.11.1 the description of the I calendar pattern format is as follows:

The 'I' symbol must not be used with any other symbol with the exception of 'escape for text'. It
represents calendar formats that match those defined in the restricted profile of the ISO 8601
standard proposed by the W3C at http://www.w3.org/TR/NOTE-datetime. The formats are
referred to as 'granularities'.
• xs:dateTime. When parsing, the data must match one of the granularities. When
unparsing, the fullest granularity is used.

Referring to http://www.w3.org/TR/NOTE-datetime give us the fullest granularity for a dateTime of:

YYYY-MM-DDThh:mm:ss.sTZD

With the description of TZD as:

TZD  = time zone designator (Z or +hh:mm or -hh:mm)

This profile defines two ways of handling time zone offsets:

    Times are expressed in UTC (Coordinated Universal Time), with a special UTC designator ("Z").
    Times are expressed in local time, together with a time zone offset in hours and minutes. A time zone offset of "+hh:mm" indicates that the date/time uses a local time zone which is "hh" hours and "mm" minutes ahead of UTC. A time zone offset of "-hh:mm" indicates that the date/time uses a local time zone which is "hh" hours and "mm" minutes behind UTC.

So it is quite clear that I should support both the Z timezone designator and the +hh:mm format. But it is not clear which should be used on output for a datetime in UTC.