Week numbers are hard.

weeknumbers.java

// Week numbers are hard. Here's a bug I found earlier today.

// If you want to construct a string like this
    String yearWeekKey =  year + week;
// then you can't use
    String year = new SimpleDateFormat("yyyy").format(date);
    String week = String.format("%02d", new LocalDate(date).getWeekOfWeekyear()); 
// when date is 2016-01-01 because the key will be "201653" and that week does not exist. 
// Instead you have to use .weekyear() because that correctly returns 2015 for 2016-01-01.
    String year = new DateTime(ets).weekyear().getAsString();
// Result "201553"

// PS. Joda-Time and ISO 8601 applies DS.