Both leaders would have a button on war page thats basically "ready!" If one leader pushes it and confirms his clan is ready, then a notification is sent to other leader, if the other leader chooses to also push the ready button, then war begins, if he chooses to wait until his clan is ready, be it an hour, two, or 10, when he pushes it, war starts. If he doesn't push it, then war starts as scheduled, after the full 24 hour prep. day.

