Two things to note here
1)
The if condition should be between the app variable and the object Datum. You are trying to retrieve the object that has Datum = app variable which is not available. So it should be
App AktuellesDatum = get property Datum of object j
The default date format of Airtable is date and time together so first check the content of Datum in object and make sure it is in similar format to your app variable.
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