How to deal with 'Inconsistent date types'?

Example:

In the column "Date" all dates are saved in the German format "DD.MM.YY" except for the date in row 8 which is saved in the format "YYYY-MM-DD" therefore giving an "Inconsistent date format" error

How to fix:

Click on a different cell in the same column/row (which has the correct format), select the format painter, and click on the highlighted cell with the inconsistent value.

Alternatively, adjust the date type directly in the "Format cells" settings:

(right-click on the cell -> )

Return to the list of inconsistencies