I am trying to display the difference between a start and end time in hh:mm:ss. The calculation doesnt seem to work properly.

I used the following code:

TimeDiff = 
VAR FirstDateStart =
    MINX ( unit_task_timestamps, unit_task_timestamps[NEW] )
VAR LastDateEnd =
    MAXX ( unit_task_timestamps, unit_task_timestamps[TASK_STATUS_ENROUTE] )
RETURN
    FORMAT ( LastDateEnd - FirstDateStart, "hh:mm:ss" )

Below is an image of the result: