diff --git a/src/index.js b/src/index.js
index 112ae33b0..a80fcd3ac 100644
--- a/src/index.js
+++ b/src/index.js
@@ -232,7 +232,8 @@ class Dayjs {
     number = Number(number) // eslint-disable-line no-param-reassign
     const unit = Utils.p(units)
     const instanceFactory = (u, n) => {
-      const date = this.set(C.DATE, 1).set(u, n + number)
+      // clone is for badMutable plugin
+      const date = this.clone().set(C.DATE, 1).set(u, n + number)
       return date.set(C.DATE, Math.min(this.$D, date.daysInMonth()))
     }
     const instanceFactorySet = (n) => {
@@ -343,7 +344,8 @@ class Dayjs {
   }
 
   daysInMonth() {
-    return this.endOf(C.M).$D
+    // clone is for badMutable plugin
+    return this.clone().endOf(C.M).$D
   }
 
   $locale() { // get locale object
diff --git a/src/plugin/badMutable/index.js b/src/plugin/badMutable/index.js
new file mode 100644
index 000000000..d8c362400
--- /dev/null
+++ b/src/plugin/badMutable/index.js
@@ -0,0 +1,26 @@
+export default (o, c) => { // locale needed later
+  const proto = c.prototype
+  proto.$g = function (input, get, set) {
+    if (this.$utils().u(input)) return this[get]
+    return this.$set(set, input)
+  }
+
+  proto.set = function (string, int) {
+    return this.$set(string, int)
+  }
+
+  const oldStartOf = proto.startOf
+  proto.startOf = function (units, startOf) {
+    this.$d = oldStartOf.bind(this)(units, startOf).toDate()
+    this.init()
+    return this
+  }
+
+  const oldAdd = proto.add
+  proto.add = function (number, units) {
+    this.$d = oldAdd.bind(this)(number, units).toDate()
+    this.init()
+    return this
+  }
+}
+
diff --git a/test/plugin/badMutable.test.js b/test/plugin/badMutable.test.js
new file mode 100644
index 000000000..e2413df84
--- /dev/null
+++ b/test/plugin/badMutable.test.js
@@ -0,0 +1,150 @@
+import MockDate from 'mockdate'
+import moment from 'moment'
+import dayjs from '../../src'
+import badMutable from '../../src/plugin/badMutable'
+
+dayjs.extend(badMutable)
+
+beforeEach(() => {
+  MockDate.set(new Date())
+})
+
+afterEach(() => {
+  MockDate.reset()
+})
+
+describe('Set', () => {
+  it('Setters', () => {
+    const d = dayjs()
+    const m = moment()
+    expect(d.year()).toBe(m.year())
+    d.year(2000)
+    m.year(2000)
+    expect(d.format()).toBe(m.format())
+    d.month(1)
+    m.month(1)
+    expect(d.format()).toBe(m.format())
+    d.day(1)
+    m.day(1)
+    expect(d.format()).toBe(m.format())
+    d.date(1)
+    m.date(1)
+    expect(d.format()).toBe(m.format())
+    d.hour(1)
+    m.hour(1)
+    expect(d.format()).toBe(m.format())
+    d.minute(1)
+    m.minute(1)
+    expect(d.format()).toBe(m.format())
+    d.second(1)
+    m.second(1)
+    expect(d.format()).toBe(m.format())
+    d.millisecond(1)
+    m.millisecond(1)
+    expect(d.format()).toBe(m.format())
+  })
+
+  it('Set', () => {
+    const d = dayjs()
+    const m = moment()
+    d.set('year', 2000)
+    m.set('year', 2000)
+    expect(d.format()).toBe(m.format())
+    d.set('month', 12)
+    m.set('month', 12)
+    expect(d.format()).toBe(m.format())
+    d.set('day', 1)
+    m.set('day', 1)
+    expect(d.format()).toBe(m.format())
+    d.set('date', 1)
+    m.set('date', 1)
+    expect(d.format()).toBe(m.format())
+    d.set('hour', 1)
+    m.set('hour', 1)
+    expect(d.format()).toBe(m.format())
+    d.set('minute', 1)
+    m.set('minute', 1)
+    expect(d.format()).toBe(m.format())
+    d.set('second', 1)
+    m.set('second', 1)
+    expect(d.format()).toBe(m.format())
+    d.set('millisecond', 1)
+    m.set('millisecond', 1)
+    expect(d.format()).toBe(m.format())
+  })
+})
+
+describe('StartOf', () => {
+  it('StartOf', () => {
+    const d = dayjs()
+    const m = moment()
+    d.startOf('year')
+    m.startOf('year')
+    expect(d.format()).toBe(m.format())
+    d.startOf('month')
+    m.startOf('month')
+    expect(d.format()).toBe(m.format())
+    d.startOf('day')
+    m.startOf('day')
+    expect(d.format()).toBe(m.format())
+    d.startOf('date')
+    m.startOf('date')
+    expect(d.format()).toBe(m.format())
+    d.startOf('hour')
+    m.startOf('hour')
+    expect(d.format()).toBe(m.format())
+    d.startOf('minute')
+    m.startOf('minute')
+    expect(d.format()).toBe(m.format())
+    d.startOf('second')
+    m.startOf('second')
+    expect(d.format()).toBe(m.format())
+    d.startOf('millisecond')
+    m.startOf('millisecond')
+    expect(d.format()).toBe(m.format())
+    d.startOf('week')
+    m.startOf('week')
+    expect(d.format()).toBe(m.format())
+  })
+})
+
+describe('Add', () => {
+  it('Add', () => {
+    const d = dayjs()
+    const m = moment()
+    d.add(1, 'year')
+    m.add(1, 'year')
+    expect(d.format()).toBe(m.format())
+    d.add(12, 'month')
+    m.add(12, 'month')
+    expect(d.format()).toBe(m.format())
+    d.add(1, 'day')
+    m.add(1, 'day')
+    expect(d.format()).toBe(m.format())
+    d.add(1, 'date')
+    m.add(1, 'date')
+    expect(d.format()).toBe(m.format())
+    d.add(1, 'hour')
+    m.add(1, 'hour')
+    expect(d.format()).toBe(m.format())
+    d.add(1, 'minute')
+    m.add(1, 'minute')
+    expect(d.format()).toBe(m.format())
+    d.add(1, 'second')
+    m.add(1, 'second')
+    expect(d.format()).toBe(m.format())
+    d.add(1, 'millisecond')
+    m.add(1, 'millisecond')
+    expect(d.format()).toBe(m.format())
+    d.add(1, 'week')
+    m.add(1, 'week')
+    expect(d.format()).toBe(m.format())
+  })
+})
+
+it('daysInMonth', () => {
+  const d = dayjs()
+  const m = moment()
+  expect(d.daysInMonth()).toBe(m.daysInMonth())
+  expect(d.format()).toBe(m.format())
+})
diff --git a/types/plugin/badMutable.d.ts b/types/plugin/badMutable.d.ts
new file mode 100644
index 000000000..30ec75e5d
--- /dev/null
+++ b/types/plugin/badMutable.d.ts
@@ -0,0 +1,4 @@
+import { PluginFunc } from 'dayjs'
+
+declare const plugin: PluginFunc
+export = plugin