This datatype is modeled after the calendar dates defined in Chapter 5.2.1 of ISO (International Organization for Standardization) 8601. Its value space is the set of Gregorian calendar dates as defined by this standard; i.e., a one-day-long period of time. Its lexical space is the ISO 8601 extended format: [-]CCYY-MM-DD[Z|(+|-)hh:mm]

Valid values include: 2001-10-26, 2001-10-26+02:00, 2001-10-26Z, 2001-10-26+00:00, -2001-10-26, or -20000-04-01.

The following values would be invalid: 2001-10 (all the parts must be specified), 2001-10-32 (the days part—32—is out of range), 2001-13-26+02:00 (the month part—13—is out of range), or 01-10-26 (the century part is missing).
with an optional time zone. Time zones that aren't specified are considered undetermined.

To specify a time zone, you can either enter a date in UTC time by adding a "Z" behind the date - like this: 2002-09-24Z
or you can specify an offset from the UTC time by adding a positive or negative time behind the date - like this:2002-09-24-06:00 or 2002-09-24+06:00

The formal definition of this data type is at http://www.w3.org/TR/xmlschema-2/#date