Date.prototype.addMonth = function(addMonth) {
  var y = this.getFullYear();
  var m = this.getMonth();
  var nextY = y;
  var nextM = m;
  //如果当前月+增加的月>11 这里之所以用11是因为 js的月份从0开始
  if (m + addMonth > 11) {
    nextY = y + 1;
    nextM = parseInt(m + addMonth) - 12;
  } else {
    nextM = this.getMonth() + addMonth;
  }
  var daysInNextMonth = Date.daysInMonth(nextY, nextM);
  var day = this.getDate();
  if (day > daysInNextMonth) {
    day = daysInNextMonth;
  }
  return new Date(nextY, nextM, day);
};

//计算当前月最大天数
Date.daysInMonth = function(year, month) {
  if (month == 1) {
    if (year % 4 == 0 && year % 100 != 0) return 29;
    else return 28;
  } else if ((month <= 6 && month % 2 == 0) || (month > 6 && month % 2 == 1))
    return 31;
  else return 30;
};

Date.prototype.format = function(format) {
  var date = {
    "M+": this.getMonth() + 1,
    "d+": this.getDate(),
    "h+": this.getHours(),
    "m+": this.getMinutes(),
    "s+": this.getSeconds(),
    "q+": Math.floor((this.getMonth() + 3) / 3),
    "S+": this.getMilliseconds()
  };
  if (/(y+)/i.test(format)) {
    format = format.replace(
      RegExp.$1,
      (this.getFullYear() + "").substr(4 - RegExp.$1.length)
    );
  }
  for (var k in date) {
    if (new RegExp("(" + k + ")").test(format)) {
      format = format.replace(
        RegExp.$1,
        RegExp.$1.length == 1
          ? date[k]
          : ("00" + date[k]).substr(("" + date[k]).length)
      );
    }
  }
  return format;
};