less-butchered.js

function archiveWeeks(tasks) {
    var startOfDay = (date) => new Date(date).setHours(0, 0, 0);

    return lodash.chain(tasks)
        .reverse()
        .groupBy((task) => startOfDay(task.get('created_at'))
        .map((tasks, date) => ({tasks: tasks, date: new Date(date)}))
        .groupBy(weekOf) // because I'm lazy
        .map((days, week) => ({days: days, week: week}))
        .value();
}

function weekOf(day) {
    var sunday = new Date(day.date);
    sunday.setDate(sunday.getDate() - sunday.getDay());

    var saturday = new Date(sunday);
    saturday.setDate(saturday.getDate() + 6);

    return formatDate(sunday) + ' - ' + formatDate(saturday);
}

original.js

function archiveWeeks(tasks) {
    var weeks = lodash.chain(tasks)
        .reverse()
        .groupBy(dayOf)
        .map(toDayObj)
        .groupBy(weekOf)
        .map(toWeekObj)
        .value();

    return weeks;
}

function dayOf(task) {
    var time = task.get('created_at');
    return new Date(time.getFullYear(), time.getMonth(), time.getDate());
}

function weekOf(day) {
    var sunday = new Date(day.date);
    sunday.setDate(sunday.getDate() - sunday.getDay());

    var saturday = new Date(sunday);
    saturday.setDate(saturday.getDate() + 6);

    return formatDate(sunday) + ' - ' + formatDate(saturday);
}

function toDayObj(tasks, day) {
    return {
        date: new Date(day),
        tasks: tasks
    };
}

function toWeekObj(days, week) {
    return {
        date: week,
        days: days,
    };
}

function formatDay(date) {
    return dateformat(date, 'dddd, mmmm dS');
}

function formatDate(date) {
    return dateformat(date, 'mmmm dS');
}