js 设置当前时间的后20分钟、后一小时、前一天等等相对时间

不管是设置相对当前时间有多久时间差的时间，统一思路为：先获取当前时间的时间戳，再根据需求加减时间获得新的时间戳，然后取年月日与时分秒。实例如下：

  // 设置默认时间——先转化为毫秒数，加上 20 分钟的毫秒数，再转化回来
  setDefaultTime() {
    let t = new Date().getTime() + 1200000;
    let d = new Date(t);
    let theMonth = d.getMonth() + 1;
    let theDate = d.getDate();
    let theHours = d.getHours();
    let theMinutes = d.getMinutes();
    if (theMonth < 10) {
      theMonth = '0' + theMonth
    }
    if (theDate < 10) {
      theDate = '0' + theDate
    }
    if (theHours < 10) {
      theHours = '0' + theHours
    }
    if (theMinutes < 10) {
      theMinutes = '0' + theMinutes
    }
    let date = d.getFullYear() + '-' + theMonth + '-' + theDate
    let time = theHours + ':' + theMinutes
    let Spare = date + ' ' + time
    console.log(date)
    console.log(time)
    console.log(Spare)
  },