Hmm, Not sure how or why you would want to use clock time as total hours. I dont see an easy, logical way of completing this task. There are two items here. 1. You want to calculate time, but using clock hours. Logically, this is not possible. BUT, you can make it LOOK like it is.
I would convery the time for decimal, do the math, then the answer can be converted to HH:MM using a format and concatenation. I did something similar in Excel. Users can type 0000, 0115, 0345, 1600, 1845, etc and it will calculate the total hours per day. Only issue is I used 15 culomns to get from Clock time to decimal hours. Standard Excel has no conversion for what I needed. If you require a more explicit explanation, please send an email to eparisek@eparisek.com along with the database, with or without the data, or partial data...and we can work together on this.
Hope all else helps and good luck!