You can only have one formula per cell.
If you format three cells as times then you can e.g. enter start time in A1, finish time in B1 and in C1 enter
to get your monthly total you need an extra column, say D. In D1 enter
Hover the cursor over the bottom right corner of D1 and it will turn into a +
Keep the left hand mouse button pressed and pull the mouse down to copy this formula down as far as you need to. If you then have a look you will see e.g. in D2 the copied formula has become
in other words it will keep a running total.