public static SortedMap<Date, Set<Task>>
                    calendar(Iterable<Task> tasks, Date start, Date end) {
        SortedMap<Date, Set<Task>> map = new TreeMap<Date, Set<Task>>();

        for (Task task : tasks) {
            Date startTime = task.getTime();
            Date endTime = task.getEndTime();

            for (Date pt = (Date) startTime.clone(); !pt.after(endTime);
                    pt.setTime(pt.getTime() + task.getRepeatInterval() * 1000)) {
                if (!map.containsKey(pt)) {
                    Set<Task> set = new HashSet<Task>();
                    set.add(task);
                    map.put((Date) pt.clone(), set);
                } else {
                    Set<Task> set = map.get(pt);
                    set.add(task);
                }
            }
        }
        return map;
    }