sum time from DB

Question

Hi all,
I have a DB table with :
time_date     Object
10:10:10       0
11:05:00       0
12:12:12       1
15:10:10       0
17:10:10       1
17:33:33       1
18:00:00       0
18:55:55       1
19:20:22       0
.....
etc. .....
I need to sum working hours when I have 0 in the second column.
Help me please!
 
Thanks and Regards,
Triphon.