The ‘if’ node in your screenshot will run 7 times independently, once for each item. If you want to compare the 6 items in the lower branch to the 1 item in the upper branch, you need a ‘merge’ node before the ‘if’ node. Set the ‘merge’ node to Combine → All possible combinations, assuming there will only ever be a single item in the upper branch. That should output 6 items, each with one date from the lower branch and the date from the upper branch included.
Would be great if you could share your workflow (or a simplified version) as gua suggested if you need any more help.