When I sum a column the result is a date in the results cell!

When I sum a column containing only numbers (formatted numbers) I get a future date in the selected result cell at the bottom of the column. There are no dates in that column. Help --- spent hours on this already. Only every couple years do I need to prepare a 300+ row spreadsheet, with column headings for date, item, hours, etc, each formatted for type of entry.

I tried deleting the one work text heading in the header cell and formatting that cell for numbers, but leaving it blank, but no matter what the end result is some date in Nov 2015.