Formulas for a unique timesheet

I previously created a timesheet for myself that allowed me to add tasks throughout the day with the amount of time used for that task. It would automatically add to a cell that kept the overall time of day accounted for. In other words, if I started on a task at 8am and worked until 8:40, I would add .7 to the task (equal to 42 minutes) and the cell at the top would now say 8:42am.

The reason this approach was helpful to me was it helped me account for every minute during the day. If I checked the sheet at 9:30am (actual time), but the sheet said 8:42am, I know I had to account for those 48 minutes with which tasks I was just working on.

Unfortunately, I have lost that time sheet and am struggling to find the right formulas to get it working again.

Here is an example of what I am trying to build... and what I have been able to reconstruct: http://screencast.com/t/dGwQTZeMqXsB

Thanks in advance for your help! This can be done... I just can't remember how I originally did it.