Yii :: Adding Create date and Update Time Automatically

Yii-TimeStamp

/**
 * @return array validation rules for model attributes.
 */
public function rules()
{
    return array(
        array('title','length','max'=>255),
        array('title, created, modified', 'required'),
        array('modified','default',
              'value'=>new CDbExpression('NOW()'),
              'setOnEmpty'=>false,'on'=>'update'),
        array('created,modified','default',
              'value'=>new CDbExpression('NOW()'),
              'setOnEmpty'=>false,'on'=>'insert')
    );
}

How can i use date('Y-m-d H:i:s', time()) instate of new CDbExpression('NOW()').
Because i want to update time of my time zone