code: function daysInMonth(month, year) {
    switch(month) {
      case 1:
      case 3:
      case 5:
      case 7:
      case 8:
      case 10:
      case 12:
        return 31;
      case 4:
      case 6:
      case 9:
      case 11:
        return 30;
      case 2:
        var leapyear;
        if(year % 4 != 0) {
          leapyear = false;
        } else if(year % 100 != 0) {
          leapyear = true;
        } else if(year % 400 != 0) {
          leapyear = false;
        } else {
          leapyear = true;
        }
        if(leapyear) {
          return 29;
        } else {
          return 28;
        }
      default:
        throw new Error('Unknown Month');
    }
  }
docs: Return the number of days in a month,
  taking leapyear into account.