Установите viewStartHour и viewEndHour on fly !!
var cal = Ext.ComponentQuery.query('#app-calendar')[0];
var activeView = cal.getActiveView();
activeView.body.viewStartHour = 6; //you most use an variable!!
activeView.body.viewEndHour = 12;
activeView.body.tpl.viewStartHour = 6;
activeView.body.tpl.viewEndHour = 12;
activeView.body.tpl.constructor();
activeView.body.refresh(false);
Наслаждайтесь !!!!