DateObject cannot handle UTC timezone with Unix datestamp

The following code, which I think is valid, throws an error message:

$date = new DateObject('31568460', 'UTC', 'U');
echo date_format_date($date, 'custom', 'U');

$ drush scr test.php 
WD php: Warning: DateTime::format(): The DateTime object has not been correctly initialized by its constructor in DateObject->format() (line 239 of          [warning]
/.../date/date_api/date_api.module).
DateTime::format(): The DateTime object has not been correctly initialized by its constructor in DateObject->format() (line 239 of                        [warning]
/.../date/date_api/date_api.module).