It begins D5.
Think mirror.
What if Dec 5th is point zero and Q wants us to take those 2 lists of post dates that were compiled and calculate mirror dates?
Dec. 5th = Dec. 5th …. The 12/5/17 Q post referred to 12/5/18 … zero difference.
What if Dec. 4th = Dec 6th posts. 12/4 = D5 minus 1. The mirror date would be Dec. 6th or D5 plus 1.