JS获取当前日前几天前的日期、两个日期相差天数、当月的天数、几月前的日期

查询当前日N天前的日期

    /**
     * 查询当前日N天前的日期
     * @param {-6} num 为负则为前几天，为正则为后几天
     * @param {'2020-08-01'} aDate
     */
    getDate(num,aDate) {
      var d = aDate ? new Date(aDate) : new Date();
      var date = d.getDate();
      num = num ? num : 0;
      d.setDate(date + num);
      date = d.getDate();
      var year = d.getFullYear();
      var month = (d.getMonth() + 1) < 10 ? ('0' + (d.getMonth() + 1)) : (d.getMonth() + 1);
      var day = d.getDate() < 10 ? ('0' + d.getDate()) : d.getDate();
      return year + '-' + month + '-' + day;
    },

计算两个日期相差天数

/**
* 计算2个日期相差的天数
* @param {2020-08-19} adate 
* @param {2020-08-13} bdate 
*/
dateDiff(aDate, bDate) {
// '2020-08-29'转'2020/08/29'
var s1=aDate.replace(/-/g,"/");
var s2=bDate.replace(/-/g,"/");
var d1 = new Date(s1);
var d2 = new Date(s2);
// 获取相差天数的时间戳
var time = d1.getTime() - d2.getTime();
  return parseInt(time / (1000 * 60 * 60 * 24))
}

 获取传入日期，当月的天数

    /**
     * 获取某月份的天数
     * @params {'20200801'} date
     */
    getMonthDays(date){
      var _this = this;
      var year = date.substring(0,4);
      var month = date.substring(4,6);
      var monthInt = (month.charAt(0) == '0') 
        ? parseInt(month.substring(1)) 
        : parseInt(month);
      var yearInt = parseInt(year);
      // 判断是否闰年
      var leapYear = ((yearInt % 4 == 0) || (yearInt & 100 !==0) || (yearInt % 400 ==0))
        ? true :false;
      var monthDay = new Array(12);
      monthDay[0] = 31;
      monthDay[1] = leapYear ? 29 : 28;
      monthDay[2] = 31;
      monthDay[3] = 30;
      monthDay[4] = 31;
      monthDay[5] = 30;
      monthDay[6] = 31;
      monthDay[7] = 31;
      monthDay[8] = 30;
      monthDay[9] = 31;
      monthDay[10] = 30;
      monthDay[11] = 31;
      return monthDay[monthInt - 1]
    },

计算N月前或后的当天日期

如2020-03-31，num为-1，为1月前日期2020-02-29

如2020-03-31，num为1，为1月后日期2020-04-30

    /**
     * 计算N月后的日期
     * @param {3} num 可为正负
     * @param {'2020-08-01'} date 
     */
    getMonth(num,date){
      if(date.length>8){
        date = date.split('-').join('')
      }
      var _this = this;
      var year = date.substring(0,4); // 2020
      var month = date.substring(4,6); // 08
      var day = date.substring(6,8); // 01
      var dayInt = (day.charAt(0) == '0') 
        ? parseInt(day.substring(1)) 
        : parseInt(day); // 1
      var monthInt = (month.charAt(0) == '0') 
        ? parseInt(month.substring(1)) 
        : parseInt(month); // 8
      var yearInt = parseInt(year); // 2020
      var newYear = yearInt; // 2020
      var newDate = '';
      var newMonth = monthInt; // 8
      var newDay = dayInt; // 1 
      var months = monthInt + num; // 8 + num
      var dateVar = date; // 2020-08-01
      var monthDays = 0; 
      if (num >= 0) { // 日期往后
        while (months > 12) {
          months -=12;
          newYear++
        } 
      } else {
        while (month < 1) {
          months +=12;
          newYear--
        }
      }
      newMonth = months;
      dateVar = (newYear + '') + (newMonth < 10 
        ? ('0' + newMonth) 
        : ('' + newMonth) + '01'); // 20201001
      monthDays = this.getMonthDays(dateVar);
      if (dayInt < monthDays) {
        newDay = dayInt;
      } else {
        newDay = monthDays;
      }
      newDate = (newYear + '') + '-' + (newMonth < 10 
        ? ('0' + newMonth) 
        : ('' + newMonth))
        + '-' + (newDay < 10 ? ('0' + newDay) : newDay);
      return newDate;
    },

做个记录，随去随用。