FYI - To add support to the original poster - I've had the same problem. I'm an Excel expert and pretty much know all/most of the features and formats. However, there's a large spreadsheet that I open in compatibility mode where Excel has set the default format for all cells to be a date format. So if I go to an empty cell and type in "=2+3", Excel will show whatever the date equivalent is to "5" as the result in the cell. I don't think I pressed a random key combination, but it's pretty annoying/frustrating.