Excel is a very smart application and is trying to help you. Excel does not distinguish between dates and numbers. If you are doing a sum of a list of numbers in a column and this column contains a date. Excel perceives this date as a number and supposes that you should include this date in your sum. for that reason your sum cell is highlighted with a green box and an exclamation mark. You can just click on the exclamation mark in the excel cell and select ignore the error.
