butchered.js

function archiveWeeks(tasks) {
    return lodash.chain(tasks)
        .reverse()
        .groupBy({created_at} => new Date(created_at.getFullYear(), created_at.getMonth(), created_at.getDate()))
        .map((t, d) => {date: new Date(d), tasks: t})
        .groupBy(weekOf) // because I'm lazy
        .map((d, w) => {week: w, days: d})
        .value();
}

function weekOf(day) { // pretend it's inline
    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');
}