I've read similar problems on the Knowledgebase.

Our different systems use US and UK date formats, so I like to format the
whole column to read as '14-Mar-01' to avoid confusion.

But looking within a cell, it might have been downloaded as 06/05/07 (it's a
real problem in these middle months of the year) but I know it should be
05/06/07

Is there a formula that I could put in a cell alongside, which I could then
copy down the hundreds of other rows for the monthly (or yearly data)?