Laravel 5 date_format validation does not validate

here is input value : FIRST_TIME_ARRIVAL_DATE_AT_DAW : 2016-21-5 17:21:35 here is validation rule :

public static $createRules = [
      'FIRST_TIME_ARRIVAL_DATE_AT_DAW' => 'date_format:"Y-m-d H:i:s"',
];

and it doesn't validatate, it return :

{"CELLS_EXPIRY_DATE":["The FIRST TIME ARRIVAL DATE AT DAW does not match the format Y-m-d H:i:s."]

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