Mean time calculation

Hi friends, My Data looks like


A B C
01/01/2015 21:23:01 0

01/01/2015 09:23:01 1 01/01/2015 09:23:01

01/01/2015 08:23:01 0

01/01/2015 07:23:01 1 01/01/2015 21:07:01

01/01/2015 21:23:01 0


means, whenevr the value in column 'B' is 1, then relevant column 'A' data will be transfered to column 'C'
so, my worry is to calculate the mean time between the two dates in the column 'C' with the static formula, (I can't use the normal cell minus formula)

and sometimes, I may get only one datetime value in the column 'C' as condition is false in column 'B'.

Please help me to solve this, I tried my level best, but i got exhausted.

please find the attached picture, where you can understand well, if this is not understandable

Thanks
Ganesan