If you are having issues when using the SUM() function with time values in Google Spreadsheet, then please make sure you are using the format Hours and not Time in your cells.
Then, try adding the new sum cell using for example SUM(A1:A10) when A1 to A10 means the column/rows containing the values to sum.
This could be an usual problem while dealing with timesheets or time tracking on a Google Spreadsheet.
