Try this calculation ;
IIF(ATTR([DATASET- 2].[DATE]) < ATTR([DATE]) , ATTR([DATE]) , ATTR([DATASET-2].[DATE])).
This says that if my Dataset 2 date is lees than my date then give the Date else give the Dataset-2 Date. this will work with a true-false policy. please check if this works.
thanks for the idea!
Unfortunately, that won't solve it as only one of the if-conditions is met...