Java / Android - Calculate difference between timestamps

I have a problem figuring out how to calculate the elapsed time between two timestamps (hours, minutes, seconds). This is the result:

String starttime = beginElement.getTimestamp();
String endtime = element.getTimestamp();

I/Start time: 1496258892612

I/End time: 1496258928999

long diffTime = Long.parseLong(endtime) - Long.parseLong(starttime);

I/Diff time: 36387

String elapsedtime = new SimpleDateFormat("hh:mm:ss").format(Long.parseLong(String.valueOf(diffTime)));

I/Elapsed time: 01:00:36

In fact, the elapsed time is just 36 seconds. It's always one hour less than the result.

How do I fix this strange issue?