First of all can you make sure that columns A, B and C are set to "Time" so they display as HH:MM:SS.
Your formula for the C column should be B subtract A to get the difference
Then, the total of C needs to be =SUM(C<startcell>:C<endcell>)
this will give you the correct total.
if you wish to change the format of the cells from HH:MM:SS then you will need to select "Custom" and select HH:MM.