Tried this:
For an attribute I created this formula:
IIf([Anfang]>DateSerial(2004;5;10)AND[Anfang]<DateSerial(2004;6;2);"Ja;"Nein")
MS Project complained, this formula is not valid (I am using the German version).
I didnt type in the formula, I only used the buttons to create it. In the Help...