Help me understand how to format time in Numbers


I am trying to sum together time values (HH:MM) on a Numbers spreadsheet and express it as a time value (HH:MM) but I don't understand how to write the function in the total cell. No matter what I do, I keep getting an error exclamation. Please help me figure this out.
I can't enter data into the Total cell because it needs to Sum from the rows above. That's what I can't figure out.
Thanks for the link. However, I keep trying to change the cell to Duration but it keeps changing it back to date & time.
If you have just recently _upgraded_ both your OS X system, and the Numbers app, then I would offer the suggestion to log in to a different user account, and try to set up your formulas again. Create a fresh user account just for this test.
I don't know what other meaning you would take from that.
To rephrase: Log in to a different user account (a newly created account will probably eliminate possible "carryover" errors from a system update. Then launch Number, and create a new document, using your particular ideas about how to set up the sum of time data.
From what I read, you can't sum formatted time. It would need to be formatted as duration data first (as in the link that I found)
Perhaps that would work differently in Mavericks and Numbers (which I don't have available).
If your duration data continues to revert to time format, then you have some other situation (which would need a different solution, I suppose)