Answer by Dominique for EXCEL IF AND Formula in excel to not calculate any date 31/12

I think this should be your formula:

=IF(AND(OR(DAY(G6)<>31;MONTH(G6)<>12);OR(I6="AT";I6="CH";I6="DE";I6="NL";I6="SE"));"Keep";"Delete Line")        ^^        ^^ (OR instead of AND)

You have forgotten that: NOT(A AND B) = (NOT (A)) OR (NOT (B)).