Timestamp conversion to date, time and duration calculation

I have scheduled tasks table where I store scheduled task id, type, exec_time, end_time etc...

I get the exec/end time as timestamp.

For BI purposes I need to somehow calculate the duration (end_time - exec_time) and return it in hh:mm:ss.

The format of the exec/end_time is dd/mm/yy hh:mm:ss:miliseconds o'clock.

Please help me with this. Thanks in advance! Steve