http://www.w3.org/TR/xmlschema-2/#dateTime

In the XML-Schema 2 specification, the datatype dateTime has several
aspects that our current UNO API css::util::DateTime struct cannot represent:

1. negative values
An XMLSchema-2 dateTime may have an "-" prefix to denote negative dates (BCE).
util::DateTime has all unsigned members, and no sign field.

2. fractional second precision
XMLSchema-2 specifies that fractional seconds must be supported with
at least 3 decimal digits of precision.
util::DateTime only has HundredthSeconds precision.

3. timezones
An XMLSchema-2 dateTime may contain a timezone offset.
util::DateTime has no timezone support.
It would be possible to convert all timezoned values to UTC on import;
however, timezoned values must sill be distinguishable from non-timezoned
values, which are specified to be in some unknown timezone, and thus are
not in general comparable with timezoned values.