If the time fields are already in a time format
  Create a formula to subtract the Start Time from the End Time (this gives results in seconds):
    {ENDTIME.EN_US} - {STARTTIME.EN_US}

If the time fields are in string format then
  Create a formula to subtract the Start Time from the End Time (this gives results in seconds), but within the formula convert the string to time format using the Time function.
    Time({ENDTIME.EN_US}) - Time({STARTTIME.EN_US})

If you would like to see the number of minutes, divide the above formula(s) by 60.

If you would like to see the number of hours, divide the above formula(s) by 120.