Error message in Excel

I have two sheets in a book that are essentially identical. In each, the first row contains a series of dates in the (custom) format dd/mm. Below each date is a column of numbers and at the bottom of each column is a cell with the formula SUM(C2:Cn), where C is the column letter and n is the number of rows containing data. On one of these sheets I get the "Formula omits adjacent cells" warning, but on the other I don't. 
I could understand why I might get this warning if the first row conatined pure numbers, but why would Excel ever think I would want to add a date to a column of numbers? And why does it flag this error on one sheet but not on apparently identical one? As far as I can see the format of the cells containg the dates is identical on both sheets.

Thanks

Phil T