Прибавить секунду к дате?

<?php

$interval = new DateInterval('PT1S'); 
$start = new Datetime('2016-05-24 00:00:01');
 
$period = new DatePeriod($start, $interval, 3 * 24 * 60 * 60);
foreach ($period as $date) { 
    echo $date->format('d-m-y | H:i:s') . PHP_EOL; 
}

<?php

$interval = new DateInterval('P1D'); 
$start = new Datetime('2016-05-24');
 
$period = new DatePeriod($start, $interval, 3);
foreach ($period as $date) {
    $day = $date->format('d-m-y');
    for ($H = 0; $H < 24; $H++) {
        for ($i = 0; $i < 60; $i++) {
            for ($s = 0; $s < 60; $s++) {
                echo sprintf('%s | %02d:%02d:%02d', $day, $H, $i, $s) . PHP_EOL; 
            }
        }
    }
}