First, add two columns next to Date, the first showing format DateTime, the second showing date in the UTCtime.
Second, have a look at the database-content. You will probably see the dates being set in the database using UTC and probably one day earlier because of the difference in timezone since your excel is German.
That explains the last of the four dates. Why the first three end up with the correct date is actually a bigger mystery. Although Ronald Caterels correctly points out that the first three are dates in the summertime.
What is the date setting in your project at Project settings->Language->edit Language->Date format. Or are you having a date parser at the Date-field?