This is somewhat what your tables seems to have - a Start Time, End Time, Total Hours, Task 001, Task 002, Task 003, ST and OT - nothing new here.
What you see on the first 4 columns (from range C->F) is basically how I thought regarding getting the difference of hours for the field Total Hours. On column C and D you have the usual military time (0800, 1600, 0800, 1900). To have theses values show up as correct military time (and to prevent errors in the future) you have to select the cells (in this case C3:D4 and then go to Home->Number->Format as Text). The problem here (or at least I saw it as one) is that Excel doesn't automatically detect that those numbers represent the military time so you have to somehow tell to the program that those numbers represent hours. That is what columns E and F are doing , they are taking in the values from the previous cells and "transforming" then in actual time. Here is the formula* that I've used for E3,E4,F3,F4:
Applying that formula to each field will net you the values that you see in columns E and F. This is good, we now have the correct time! So the column H is basically the difference between the Start Time and End Time. In cells H3 and H4 you'll find the following (simple) formula**:
If we move to column M, this is the formula I've used in cell M3, M4:
In cells N3,N4 we have the following code:
Let me know if this helped you in any way. Like I said I love Excel and see it as a really powerful tool but sometimes people think that it's too complicated to make certain things - believe me, it is not. I took more time writing you this reply than making the example in Excel, just to give you an idea that for a average user this is a simple thing.
Let me know if I can help you any further!
* If you want the process that I've used to build these formulas, but for now I'll just post what I built
** I've multiplied by 24 to have a result in hours and minutes ( say 2.5 represent 2 hours and 30 minutes)