All you need to do is make Dio suck out Jostar's blood, it will add 3 seconds more, this is not a bug, and only works if "the world" is active when Dio is sucking out Jostar's blood, and only works on the "the world" that was active. After you spawn another "the world", it will revert back to 7 seconds. You can keep adding 3 seconds if you follow the instructions.
the Dev already said this but i want to clarify anyways.