Problem with sorting dates

In spreadsheet I have a column of dates near the end of rows of other data. I have formatted them all the same but when I try to sort all data by the dates in that column there’s a group of dates that are not integrated properly in the results. Any suggestions?

Update: I’ve discovered that when I click on a cell that is properly sorted I see “Sum = 12/25/2012” at the bottom of the spreadsheet. When I click on a cell that hasn’t been sorted properly I see “Sum =0”.

I discovered the dates that weren’t sorting properly had an apostrophe before them that was only visible in the input line. Once I removed that from the cells they sorted properly.