$dateFormatIso = 'yyyy-M-d HH:mm:ss';
        $fieldset->addField(
            'start_time', 'date', array(
            'name'         =>   'start_time',
            'time'         =>    true,
            'label'        =>    Mage::helper('adminhtml')->__('Start Time'),
            'title'        =>    Mage::helper('adminhtml')->__('Start Time'),
            'image'        =>    $this->getSkinUrl('images/grid-cal.gif'),
            'input_format' =>    Varien_Date::DATETIME_INTERNAL_FORMAT,
            'format'       =>    $dateFormatIso,
                                                  )
        );