Excel 2007 : Is it possible to undo automatic conversion of numbers into dates?

Hi, I'm experiencing the following problem:

I pasted data from an online database into excel and it automatically converted some stock prices and percentages into dates, such as 04.09.2009 or 01.12.1975.

This I believe happened only for numbers that had one component within the range 1 to 12. So for example sth like 24,63 was not converted, but sth like 12,63 would be converted.

Now, my subscription to the dataset has expired, and I only just noticed that excel converted these stocks into dates.

Is there any way to convert the dates back into numbers? Perhaps there is some unique rule according to which excel converted the stock prices into dates? Any suggestions welcome!