code: function getWeekNumber(day) {
    var date = clone(day);
    date.setHours(0, 0, 0);
    date.setDate(date.getDate() + 4 - (date.getDay() || 7));
    return Math.ceil(((date - new Date(date.getFullYear(), 0, 1)) / 8.64e7 + 1) / 7);
  }
docs: Return the year's week number (as per ISO, i.e. with the week starting from monday)
  for the given day.
  
  @export
  @param {Date} day
  @returns {Number}