You can put a formula in the "total time" cell. It would be something like this:

If the start time for Task 1 on March 17 is in cell D7 and the finish time is in cell E7, then the formula would be "=e7-d7" . This will automatically calculate the amount of time spent on Task 1 on March 17.

Then you can just use "edit, fill down" to copy the formula into the remaining cells.

I hope this answers your question...