How to sum up hours in a sheet/table?

Question

Hi!
I logged into my ISP's pages to check telephone calls in previous month.
The list of calls can be exported to an Excel spreadsheet which I did.
But when opening the sheet in Numbers '09, under duration column each cell shows time in hh:mm:ss format but when clicked it shows dd/mm/yyyy hh:mm:ss format, and when trying to apply Sum function at the bottom of the list, all I get is a red triangle with a white exclamation mark inside saying "The formula's arguments can only include one date value."
What should I do to remove dates from each cell, leaving only hours, minutes, seconds and finally add all up at the bottom with the total time?
Thank you

Answer 1

Herebelow are shown the first 10 rows of the column "Duration / min" I'd like to arrange by summing up time.
I applied the following formatting:
opened Inspector, Cells, Date and Time, Date shown as "dd/mm/yyyy", Time shown as "hh:mm:ss"
The incorrect date (19th century) is shown most probably because by default it's an Excel spreadsheet.
31/12/1899 00:05:18
31/12/1899 00:01:43
31/12/1899 00:00:19
31/12/1899 00:10:16
31/12/1899 00:00:20
31/12/1899 00:02:39
31/12/1899 00:00:10
31/12/1899 00:00:10
31/12/1899 00:00:10
31/12/1899 00:00:54
Is there any way to remove date from the cells in order to nicely sum up the time values?
Thanks