$time = '11:00:00';
$duration = '03:30:00';
$newTime = strtotime($time) + strtotime($duration);
$newTime = date("H:i:s", $newTime);
echo $newTime;
количество секунд, прошедших с 1 января 1970 года 00:00:00 UTC
$time = '11:00:00';
$duration = '03:30:00';
$timeParts = explode(':', $time);
$durationParts = explode(':', $duration);
function timeSum(...$arParams) {
return str_pad(array_sum($arParams), 2, '0', STR_PAD_LEFT);
}
$newTimeParts = array_map('timeSum', $timeParts, $durationParts);
$newTime = implode(':', $newTimeParts);
echo $newTime;