For this theory to work. The killer would need to know the birthdays of all the victims. It would also mean thy were not targets of opportunity but carefully researched targets.

Could just be as simple as the killer having some obsessive compulsion with numbers and wanting to kill on dates or times that added to something that had some mathematical pattern or meaning to him.

The Ray Davis Murder in Oceanside is possibly now linked to Zodiac – 4/9/62 would add to 75