docs: function week(date) {
    var d = new Date(+date);
    var y, w;
  
    d.setHours(0, 0, 0);
    d.setDate(d.getDate() + 4 - (d.getDay()||7));
  
    y = new Date(d.getFullYear(), 0, 1);
    w = Math.ceil((((d - y) / DAY_M) + 1) / 7);
    return d.getFullYear() + '-W' + zeroFill(w);
  }