Finding difference between date now and last comment
Current time is easily obtained simply with new DateTime(), no need to specify any formatting (default is now). Also you don't need to format dates to compare strings because DateTime itself is comparable. All together:

$now = new DateTime();
$lastPostTime = new DateTime('2016-01-08 13:16:59');
$notEnabledBefore = lastPostTime-&gt;add(new DateInterval('PT9M'));

if (now &lt; notEnabledBefore) {
    $timeToWait = notEnabledBefore-&gt;diff(now);
    echo $timeToWait-&gt;format('%i minutes left or ahead');
}