scratch the first part of step 2. when you type in the dates that way excel thinks it is text.i'm confused.... you said you wanted dd.mm.yyyy format.
regardless: it must be done in the correct order
1. Format the cell dd.mm.yyyy or dd/mm/yyyy or mm.dd.yyyy or mm/dd/yyyy It doesn't matter..... just whatever format you want.
2. Input the date 15.08.2021 or 15/08/2021 or 08.15.2021 or 08/15/2021 (according to the format above.
... if that doesnt work Just put it in with the DATE Function. =Date(2021,08,15)