Issue at importing contacts and companies with date columns

I use Excel sheets to import data from SF.

Now run into an issue what is related to the date data I will import. All dates with days between 1 and 12 have no problem. But all dates greater than 12 goes on error. 

In Excel the internal date format is used, the transformation runs into CRM 365 so my opinion. 

I have a german CRM and use german Excel, but also if I change the format to English(US) I get the same error.

Did anyone has an idea what to change to avoid this issue?

Best regards

Ingo