code: function distance (dt1, dt2) {
    let dtBegin, dtEnd
    if (dt2) {
      dtBegin = Miment(dt1).valueOf()
      dtEnd = Miment(dt2).valueOf()
    } else {
      dtBegin = this.valueOf()
      dtEnd = Miment(dt1).valueOf()
    }
    let m = Miment(dtBegin - dtEnd)
    m.__distance_begin__ = dtBegin
    m.__distance_end__ = dtEnd
    return m
  }
docs: 计算两个时间距离