1. It has been a lot confusing. I read that you wanted a sum in hours / minutes. And obviously took it for a numeric value rather than a formatted one. My mistake.
2. The reason I put all those in separate columns was to give a better understanding / view of how looked and how they were worked upon.
3. Ignore the hh_mm. I wanted to say hh:mm. Was a typo.
Now that we are back at the same place where we started... I do not know why your excel complains about invalid format. For me, I just wrote down some figures in hh:mm format and at the end totalled them, My smartass German version of excel gave me the output as the input. No need of any extra effort. As here:
Note that the function Sum() is called Summe() in German (in the above image). It still is the Sum() function.
Now, to solve your problem, I suggest you try the following:
1. Sum the columns. Let it complain.
2. Right click on the cell. Select format. Select User Defined, and then on the right side select hh:mm. That should work.
If it dosen't, then unfortunately, I can't help. Maybe someone with English version of excel can help.
Regards,
Bandu.
Edit: Whats OOo?