JavaScript: DateDiff & DateMeasure: Calculate days, hours, minutes, seconds between two Dates with es6 classes

datediff.js

class DateMeasure {
  constructor(ms) {
    const negativeMS = ms < 0;
    let d, h, m, s;

    // this change is needed for it to work with
    // date1 before date2 diffs
    if (negativeMS) {
      ms *= -1;
    }

    s = Math.floor(ms / 1000);
    m = Math.floor(s / 60);
    s = s % 60;
    h = Math.floor(m / 60);
    m = m % 60;
    d = Math.floor(h / 24);
    h = h % 24;

    // this change is needed for it to work with
    // date1 before date2 diffs
    if (negativeMS) {
      d *= -1;
      h *= -1;
      m *= -1;
      s *= -1;
    }
    
    this.days = d;
    this.hours = h;
    this.minutes = m;
    this.seconds = s;
  };
}

class DateDiff {
  constructor(date1, date2) {
    this.days = null;
    this.hours = null;
    this.minutes = null;
    this.seconds = null;
    this.date1 = date1;
    this.date2 = date2;
    this.init();
  }

  init() {
    let differenceMS = this.date1 - this.date2;
    let data = new DateMeasure(differenceMS, false);

    this.days = data.days;
    this.hours = data.hours;
    this.minutes = data.minutes;
    this.seconds = data.seconds;
  };
}

// not very useful since have to input date1 again
Date.diff = function(date1, date2) {
  return new DateDiff(date1, date2);
};

// get difference of date to another date2
Date.prototype.diff = function(date2) {
  return new DateDiff(this, date2);
};