I have a problem working with some worksheets within a workbook from a 3rd party website source.
A month or so ago I got shot down when I made an aside about Excel sometimes converting UK dates that could be valid US dates to US dates. So 1/4/11 (1 April 11) would become 4/1/11 (4 January 11) but 13/4/11 (13 April 11) would be unchanged. I could not duplicate the problem so had to admit that I must have made a mistake.
I encountered this problem eight or nine years ago and since then I have always passed dates in the format "1Apr11" to avoid the problem. Your question suggested I had been correct but something other than Excel was at the root of the problem.
I set up a worksheet with numbers, dates and strings. It saved as a CSV file just as I would wish.
I went to Control Panel and changed my location to "United States". I saved the worksheet again and this time the dates in the CSV file were "Americanised" as explained above.
I restored my location to "United Kingdom". My third CSV file was again correct.
You claim that your Control Panel settings are correct. I think you need to check again and look for anything else that could be wrong since I now know how to switch this effect on and off.