This datatype describes instances identified by the combination of a date and a time. Its value space is described as a combination of date and time of day in Chapter 5.4 of ISO 8601. Its lexical space is the extended format: [-]CCYY-MM-DDThh:mm:ss[Z|(+|-)hh:mm]
The time zone may be specified as Z (UTC) or (+|-)hh:mm. Time zones that aren't specified are considered undetermined. Restrictions

The basic format of ISO 8601 calendar datetimes, CCYYMMDDThhmmss, isn't supported.
The other forms of date-times available in ISO 8601—ordinal dates defined by the year, the number of the day in the year, dates identified by calendar week, and day numbers—aren't supported.

As the value space is defined by reference to ISO 8601, there is no support for any calendar system other than Gregorian. As the lexical space is also defined in reference to ISO 8601, there is no support for any localization such as different orders for date parts or named months.

Valid values for xs:dateTime include:
-2001-10-26T21:32:52, or

The following values are invalid:
2001-10-26 (all the parts must be specified),
2001-10-26T21:32 (all the parts must be specified),
2001-10-26T25:32:52+02:00 (the hours part—25—is out of range), or
01-10-26T21:32 (all the parts must be specified).

The formal definition of this datatype is at: http://www.w3.org/TR/xmlschema-2/#dateTime