$format public property
The date format that the value being validated should follow. This can be a date time pattern as described in the ICU manual. Alternatively this can be a string prefixed with php: representing a format that can be recognized by the PHP Datetime class. Please refer to http://php.net/manual/en/datetime.createfromformat.php on supported formats. If this property is not set, the default value will be obtained from Yii::$app->formatter->dateFormat, see yii\i18n\Form