Dates change when copying...

Hello,

Thanks to this kind of forum, I've been able to deal with issues around the date systems 1900 vs 1904. It remains a problem though since I very often exchange files with correspondents in Congo who use PCs and Excel Microsoft.
But even more strange: when I copy a set of rows containing dates from one workbook to another (on my computer), the system adds two days!!? Why is that, en how can I prevent that?