Where do I set active time for war in ROC?

On the "events" tab you go to "Reign of Chaos", from there you chose the first tab "Season", on the bottom of that page is a button "Alliance Intel" go there & on that page bottom another button called "Truce Timer". Depending on when you set your truce to be active you will have your war. if you set it 00:00-08:00 then your war will be at reset +8 hours for example. Be careful because once you change the truce time you can change it again in a week.