How can I add 15 minutes to the previous event time (timestamp) to get the next event time.
Note: This event time is not in a table.
Straight forward adding the two values will probly not provide the correct result.
I can get the correct time with Mysql addtime($time, $duration) but that
is only good if the if info is in a table.

$time = "09:00:00";//timestamp hh:mm:ss
$duration = 00:15:00;//enent duration hh:mm:ss
$next_time = $event_time + $event_duration;

Thanks for the suggestion.
This is the final code that solved the problem

$event_time = 09:00:00
$event_length = 15;
$timestamp = strtotime("$event_time");
$etime = strtotime("+$event_length minutes", $timestamp);
$next_time = date('H:i:s', $etime);