ZF2 "Zend\Form\Element\Time" gives wrong message

I've got the following element in my form but on submit it keeps giving the message "The input does not appear to be a valid date". It's not a date it's a Time element. I've got no filters applied to it.

    $this->add(array(
        'name' => 'starttime',
        'type' => 'Zend\Form\Element\Time',
        'options' => array(
            'label' => _("Start time"),
            'format' => 'H:i:s'
        ),
        'attributes' => array(
            'min' => '00:00:00',
            'max' => '23:59:59',
            'step' => 60 * 15, // seconds; default step interval is 60 seconds
        )
    ));

What am i doing wrong?