The validator for date-time only allows for a datetime in UTC, however the ISO 8601 standard allows for a UTC offset to be given.
Example datetime allowed now:

2012-08-20T00:00:00Z

datetime not allowed in json-schema but is allowed by ISO 8601:

2012-08-20T00:00:00+02:00

I see from the json schema you reference that the former is the recommended datetime format, but doesn't that mean the latter should still be allowed?