No, I have no clue how to do JavaScript....and I've tried to find a pre-written JavaScript, but I do not know enough about JavaScript to understand what I found.
All I did was go into the "Time Spent" properties, I went to Calculate, and then added "Time In" and "Time Out" which is what is giving me 15, and I have no clue where it is getting the number 15 from (when I tested to see if it worked, I put in 7:00 am in the "Time In" field and 8:00 am in the "Time Out" field...and it auto-calculated 15 in the "Time Spent" field.