Hi Stan255,
Yes it's possible and it's actually now pretty easy using validation logic.
Here is an example, you need to go to the Add Logic button at the lower right side of the field you're working with, then click validation logic in the left menu and add this to the formula area. Feel free to tweak the formula if you need something like WEEKDAYDIFF instead but the general structure will work.
IF(DATEDIFF({DE: Custom Date Example}, $$TODAY) < 42, "The date must be at least 6 weeks from today.", "")