Daniel wrote:

Here's an example of what may happen with a date if it is opened by a
computer with the format (dd/mm/yyyy) or with (mm/dd/yyyy)

Original Format (dd/mm/yyyy) (mm/dd/yyyy)
6/11/2012 41219 41071
2/11/2012 41215 40950
2/11/2012 41215 40950
13/11/2012 41226 #VALUE!
14/11/2012 41227 #VALUE!
1/11/2012 41214 40919



So it is not an Excel file.

Probably you try to import text file, and depending on local settings
first date will be interpreted as June 11th or November 6th.