<?php
$array1 = [
'0' => '2018-08-21 10:00:00',
'1' => '2018-08-28 10:00:00',
'2' => '2018-09-04 10:00:00',
'3' => '2018-09-11 10:00:00',
'4' => '2018-09-18 10:00:00',
];
$array2 = [
'0' => '2018-08-21 10:00:00',
'1' => '2018-08-28 10:00:00',
'2' => '2018-09-04 10:00:00',
'3' => '2018-09-11 10:00:00',
'4' => '2018-09-18 10:00:00',
'5' => '2019-09-18 10:00:00',
];
$diff = array_diff($array2, $array1);
var_dump($array1, $array2, $diff);
preferences:
53.76 ms | 402 KiB | 5 Q