diff --git a/src/index.js b/src/index.js
index 8fb328eed..38f903f47 100644
--- a/src/index.js
+++ b/src/index.js
@@ -36,14 +36,11 @@ const dayjs = (date, c) => {
   return new Dayjs(cfg) // eslint-disable-line no-use-before-define
 }
 
-const toDayjs = input => (isDayjs(input) ? input : dayjs(input.valueOf()))
-
 const wrapper = (date, instance) => dayjs(date, { locale: instance.$L })
 
 const Utils = U // for plugin use
 Utils.parseLocale = parseLocale
 Utils.isDayjs = isDayjs
-Utils.toDayjs = toDayjs
 Utils.wrapper = wrapper
 
 const parseDate = (date) => {
@@ -99,16 +96,20 @@ class Dayjs {
     return ((this.$y % 4 === 0) && (this.$y % 100 !== 0)) || (this.$y % 400 === 0)
   }
 
-  isSame(input) {
-    return this.valueOf() === toDayjs(input).valueOf()
+  $compare(that) {
+    return this.valueOf() - dayjs(that).valueOf()
+  }
+
+  isSame(that) {
+    return this.$compare(that) === 0
   }
 
-  isBefore(input) {
-    return this.valueOf() < toDayjs(input).valueOf()
+  isBefore(that) {
+    return this.$compare(that) < 0
   }
 
-  isAfter(input) {
-    return this.valueOf() > toDayjs(input).valueOf()
+  isAfter(that) {
+    return this.$compare(that) > 0
   }
 
   year() {
@@ -339,7 +340,7 @@ class Dayjs {
 
   diff(input, units, float) {
     const unit = Utils.prettyUnit(units)
-    const that = toDayjs(input)
+    const that = dayjs(input)
     const diff = this - that
     let result = Utils.monthDiff(this, that)
     switch (unit) {
diff --git a/test/query.test.js b/test/query.test.js
index cf864dcc2..977a68e09 100644
--- a/test/query.test.js
+++ b/test/query.test.js
@@ -17,13 +17,38 @@ it('IsLeapYear', () => {
   expect(dayjs('2100-01-01').isLeapYear()).toBe(false)
 })
 
-it('Is Before Is After Is Same', () => {
-  testArr.forEach((instance) => {
-    const dayA = instance()
-    const dayB = dayA.clone().add(1, 'day')
-    const dayC = dayA.clone().subtract(1, 'day')
-    expect(dayC.isBefore(dayA)).toBe(true)
-    expect(dayA.isSame(instance())).toBe(true)
-    expect(dayB.isAfter(dayA)).toBe(true)
+describe('Is Before Is After Is Same', () => {
+  it('Compare to dayjs object', () => {
+    testArr.forEach((instance) => {
+      const dayA = instance()
+      const dayB = dayA.clone().add(1, 'day')
+      const dayC = dayA.clone().subtract(1, 'day')
+      expect(dayC.isBefore(dayA)).toBe(true)
+      expect(dayA.isSame(instance())).toBe(true)
+      expect(dayB.isAfter(dayA)).toBe(true)
+      expect(dayA.isSame()).toBe(true)
+      expect(dayB.isAfter()).toBe(true)
+      expect(dayC.isBefore()).toBe(true)
+    })
+  })
+
+  it('No value', () => {
+    testArr.forEach((instance) => {
+      const dayA = instance()
+      const dayB = dayA.clone().add(1, 'day')
+      const dayC = dayA.clone().subtract(1, 'day')
+      expect(dayA.isSame()).toBe(true)
+      expect(dayB.isAfter()).toBe(true)
+      expect(dayC.isBefore()).toBe(true)
+    })
+  })
+
+  it('With string', () => {
+    testArr.forEach((instance) => {
+      const dayD = instance()
+      expect(dayD.isSame('20180101')).toBe(false)
+      expect(dayD.isAfter('20180101')).toBe(true)
+      expect(dayD.isBefore('20180101')).toBe(false)
+    })
   })
 })