Formula for subtracting and adding timecode

Hi,

I'm working with a large volume of timecode in both 25fps and 30fps (29.97 to be exact).

It looks like this:
(Column A) (Column B) (Column C)
00:00:11:07 00:00:13:08
00:00:14:03 00:00:15:11
00:00:16:15 00:00:17:19
00:00:18:18 00:00:20:03
00:00:22:07 00:00:25:10

I need to subtract Column A from Column B to get the "duration" for each set.
Then, at the end, I need to add up all the duration timecode in Column C to get the "total duration" all in the same format, meaning HH (hour):MM (minute):SS(second):FF(frame).

I would appreciate it if someone could show me what each formula would be for getting the "duration" AND "total duration" and in different frame rate.

THANK YOU!