var date = Date.now();
var item1 = new Item({date: date, title: 'breakfast', value: 100, history: false});
var item2 = new Item({date: date, title: 'lunch', value: 200, history: false});
var item3 = new Item({date: date, title: 'dinner', value: 300, history: false});

var items = new Backbone.Collection;
items.add(item1, item2, item3);

//Create a new model derived from these models and save in collection

var sum = 0;
items.each(function(elem) {
    sum += parseInt(elem.get('value'));
    //elem.destroy();
}

var report = new Item({date: date, title: 'Report', value: sum, history: true});
items.add(report);