Check Date Format Front and Back

CheckYYYY-MM-DD.php

/**Check Date Input on Server**/
$date_regex = '/^(19|20)\d\d[\-\/.](0[1-9]|1[012])[\-\/.](0[1-9]|[12][0-9]|3[01])$/';
// Check date input
    if(isset($_POST['start'])) {
        if (preg_match($date_regex,$_POST['start']) && preg_match($date_regex,$_POST['end'])) {
            $_start = $_POST['start'];
            $_end = $_POST['end'];
        } else {
            die("Invalid Date Format");
        }
    }

<!--Check Input in Field-->
    <label for="Start" class="db">Start</label>
    <input type="date" name="start" id="Start" value="${start}"
        pattern="[0-9]{4}-(0[1-9]|1[012])-(0[1-9]|1[0-9]|2[0-9]|3[01])" />
    <label for="End" class="db">End</label>
    <input type="date" name="end" id="End" value="${end}"
        pattern="[0-9]{4}-(0[1-9]|1[012])-(0[1-9]|1[0-9]|2[0-9]|3[01])" />

Valid datetime "/^([1-2][0-9]{3})-([0-1][0-9])-([0-3][0-9])([TZ\s]{1})([0-5][0-9]):([0-5][0-9])$/"