diff --git a/package.json b/package.json
index 1404f2166..b47add5ed 100644
--- a/package.json
+++ b/package.json
@@ -10,7 +10,7 @@
     "build": "cross-env BABEL_ENV=build node build && npm run size",
     "sauce": "npx karma start karma.sauce.conf.js",
     "test:sauce": "npm run sauce -- 0 && npm run sauce -- 1 && npm run sauce -- 2  && npm run sauce -- 3",
-    "size": "size-limit"
+    "size": "size-limit && gzip-size dayjs.min.js"
   },
   "pre-commit": [
     "lint"
@@ -66,6 +66,7 @@
     "eslint-config-airbnb-base": "^12.1.0",
     "eslint-plugin-import": "^2.10.0",
     "eslint-plugin-jest": "^21.15.0",
+    "gzip-size-cli": "^2.1.0",
     "jasmine-core": "^2.99.1",
     "jest": "^22.4.3",
     "karma": "^2.0.2",
diff --git a/src/index.js b/src/index.js
index 38f903f47..d86dd056e 100644
--- a/src/index.js
+++ b/src/index.js
@@ -156,8 +156,8 @@ class Dayjs {
   startOf(units, startOf) { // startOf -> endOf
     const isStartOf = !Utils.isUndefined(startOf) ? startOf : true
     const unit = Utils.prettyUnit(units)
-    const instanceFactory = (d, m, y = this.$y) => {
-      const ins = wrapper(new Date(y, m, d), this)
+    const instanceFactory = (d, m) => {
+      const ins = wrapper(new Date(this.$y, m, d), this)
       return isStartOf ? ins : ins.endOf(C.D)
     }
     const instanceFactorySet = (method, slice) => {
@@ -171,13 +171,13 @@ class Dayjs {
     switch (unit) {
       case C.Y:
         return isStartOf ? instanceFactory(1, 0) :
-          instanceFactory(31, 11, this.$y)
+          instanceFactory(31, 11)
       case C.M:
         return isStartOf ? instanceFactory(1, this.$M) :
-          instanceFactory(0, this.$M + 1, this.$y)
+          instanceFactory(0, this.$M + 1)
       case C.W:
         return isStartOf ? instanceFactory(this.$D - this.$W, this.$M) :
-          instanceFactory(this.$D + (6 - this.$W), this.$M, this.$y)
+          instanceFactory(this.$D + (6 - this.$W), this.$M)
       case C.D:
       case C.DATE:
         return instanceFactorySet('setHours', 0)
@@ -234,37 +234,31 @@ class Dayjs {
 
   add(number, units) {
     number = Number(number) // eslint-disable-line no-param-reassign
-    // units === 'ms' hard code here, will update in next release
-    const unit = (units && (units.length === 1 || units === 'ms')) ? units : Utils.prettyUnit(units)
+    const unit = Utils.prettyUnit(units)
     const instanceFactory = (u, n) => {
       const date = this.set(C.DATE, 1).set(u, n + number)
       return date.set(C.DATE, Math.min(this.$D, date.daysInMonth()))
     }
-    if (['M', C.M].indexOf(unit) > -1) {
+    if (unit === C.M) {
       return instanceFactory(C.M, this.$M)
     }
-    if (['y', C.Y].indexOf(unit) > -1) {
+    if (unit === C.Y) {
       return instanceFactory(C.Y, this.$y)
     }
     let step
     switch (unit) {
-      case 'm':
       case C.MIN:
         step = C.MILLISECONDS_A_MINUTE
         break
-      case 'h':
       case C.H:
         step = C.MILLISECONDS_A_HOUR
         break
-      case 'd':
       case C.D:
         step = C.MILLISECONDS_A_DAY
         break
-      case 'w':
       case C.W:
         step = C.MILLISECONDS_A_WEEK
         break
-      case 's':
       case C.S:
         step = C.MILLISECONDS_A_SECOND
         break
diff --git a/src/utils.js b/src/utils.js
index 692012706..f993ae9e4 100644
--- a/src/utils.js
+++ b/src/utils.js
@@ -1,3 +1,5 @@
+import * as C from './constant'
+
 const padStart = (string, length, pad) => {
   const s = String(string)
   if (!s || s.length >= length) return string
@@ -22,9 +24,21 @@ const monthDiff = (a, b) => {
 
 const absFloor = n => (n < 0 ? Math.ceil(n) || 0 : Math.floor(n))
 
-const prettyUnit = u => (u && String(u).toLowerCase().replace(/s$/, ''))
+const prettyUnit = (u) => {
+  const special = {
+    M: C.M,
+    y: C.Y,
+    w: C.W,
+    d: C.D,
+    h: C.H,
+    m: C.MIN,
+    s: C.S,
+    ms: C.MS
+  }
+  return special[u] || String(u || '').toLowerCase().replace(/s$/, '')
+}
 
-const isUndefined = s => s === void 0 // eslint-disable-line no-void
+const isUndefined = s => s === undefined
 
 export default {
   padStart,
diff --git a/test/utils.test.js b/test/utils.test.js
index e70623e79..5a4dbaa50 100644
--- a/test/utils.test.js
+++ b/test/utils.test.js
@@ -10,7 +10,7 @@ it('PrettyUnit', () => {
   expect(prettyUnit('Days')).toBe('day')
   expect(prettyUnit('days')).toBe('day')
   expect(prettyUnit('day')).toBe('day')
-  expect(prettyUnit()).toBe(undefined)
+  expect(prettyUnit()).toBe('')
 })
 
 it('PadZoneStr', () => {