Created attachment 128720 [details] Screenshot of DATUMDIF error Function DATUMDIF result should be a number (days between first and second date) but is a date. See screenshot.
You might wanna try DATEDIF instead. https://help.libreoffice.org/Calc/DATEDIF
MM, DATUMDIF is the German localized name for DATEDIF. Thomas, can you check if you right click the cell with DATUMDIF, and select Format Cells..., which Category is selected? If it's Date, change to Number. I tried with an empty spreadsheet, and if I added the formula to an empty cell, it displayed the number correctly, but if previously Date was selected in the column, then it would display the formula result as date (it's only a display issue, changing Category to Number is a manual fix).
Aron Budea, you are right, it was Date formatted. I switched it to Numbers, now it's ok. Thanks!
All right, that seems fine, then. Probably cell format was changed to date at some point before, that's why it was displayed as one. Closing as NOTABUG.