Have you red my recent answer?? Battle day cannot start unless both sides pressed the READY-Button so the war cannot start "just a minute after prep day"!! You can see the status of your enemies, so you know whether they're ready or not. This way it won't start at unexpected times, because you can "unready" your clan for war by pressind the READY-Button twice, just like in other multiplayer gamed (e.g. CoD)
I don't feel like this would make things more complicated, basically you just add the unnecessary part of the Prep day to the battle day. CoC would become more interactive with this feature.

