Method add

src/plugin/duration/index.js:205–216

(input, unit, isSubtract)

  }

  add(input, unit, isSubtract) {
    let another
    if (unit) {
      another = input * unitToMS[prettyUnit(unit)]
    } else if (isDuration(input)) {
      another = input.$ms
    } else {
      another = wrapper(input, this).$ms
    }

    return wrapper(this.$ms + (another * (isSubtract ? -1 : 1)), this)
  }

  subtract(input, unit) {
    return this.add(input, unit, true)