Difference between duration time

In a report I have to calculate the sum of differences between the duration of two elapsed times...

Example:
Time_1 From: 09:00 Time_1 To: 10:00 = elapsed: 01:00:00
Time_2 From: 13:00 Time_2 To: 14:00 = elapsed: 01:00:00
____________________
Sum: 02:00:00

How can I have the Sum? When I try it gives me #Error ...

Thanks!