diff --git a/src/differenceInBusinessDays/index.js b/src/differenceInBusinessDays/index.ts
similarity index 80%
rename from src/differenceInBusinessDays/index.js
rename to src/differenceInBusinessDays/index.ts
index 174576b310..46fd575c74 100644
--- a/src/differenceInBusinessDays/index.js
+++ b/src/differenceInBusinessDays/index.ts
@@ -33,22 +33,22 @@ import requiredArgs from '../_lib/requiredArgs/index'
  * //=> 136
  */
 export default function differenceInBusinessDays(
-  dirtyDateLeft,
-  dirtyDateRight
-) {
+  dirtyDateLeft: Date | number,
+  dirtyDateRight: Date | number
+): number {
   requiredArgs(2, arguments)
 
-  var dateLeft = toDate(dirtyDateLeft)
-  var dateRight = toDate(dirtyDateRight)
+  const dateLeft = toDate(dirtyDateLeft)
+  let dateRight = toDate(dirtyDateRight)
 
-  if (!isValid(dateLeft) || !isValid(dateRight)) return new Date(NaN)
+  if (!isValid(dateLeft) || !isValid(dateRight)) return NaN
 
-  var calendarDifference = differenceInCalendarDays(dateLeft, dateRight)
-  var sign = calendarDifference < 0 ? -1 : 1
+  const calendarDifference = differenceInCalendarDays(dateLeft, dateRight)
+  const sign = calendarDifference < 0 ? -1 : 1
 
-  var weeks = toInteger(calendarDifference / 7)
+  const weeks = toInteger(calendarDifference / 7)
 
-  var result = weeks * 5
+  let result = weeks * 5
   dateRight = addDays(dateRight, weeks * 7)
 
   // the loop below will run at most 6 times to account for the remaining days that don't makeup a full week
diff --git a/src/differenceInBusinessDays/test.js b/src/differenceInBusinessDays/test.ts
similarity index 68%
rename from src/differenceInBusinessDays/test.js
rename to src/differenceInBusinessDays/test.ts
index fb92c4f2f0..f30e6714e1 100644
--- a/src/differenceInBusinessDays/test.js
+++ b/src/differenceInBusinessDays/test.ts
@@ -1,96 +1,97 @@
-// @flow
 /* eslint-env mocha */
 
 import assert from 'power-assert'
 import differenceInBusinessDays from '.'
 
-describe('differenceInBusinessDays', function() {
-  it('returns the number of business days between the given dates, excluding weekends', function() {
-    var result = differenceInBusinessDays(
+describe('differenceInBusinessDays', function () {
+  it('returns the number of business days between the given dates, excluding weekends', function () {
+    const result = differenceInBusinessDays(
       new Date(2014, 6 /* Jul */, 18),
       new Date(2014, 0 /* Jan */, 10)
     )
     assert(result === 135)
   })
 
-  it('can handle long ranges', function() {
+  it('can handle long ranges', function () {
+    // @ts-ignore
     if (typeof this.timeout === 'function') {
+      // @ts-ignore
       this.timeout(500 /* 500 ms test timeout */)
     }
-    var result = differenceInBusinessDays(
+    const result = differenceInBusinessDays(
       new Date(15000, 0 /* Jan */, 1),
       new Date(2014, 0 /* Jan */, 1)
     )
     assert(result === 3387885)
   })
 
-  it('the same except given first date falls on a weekend', function() {
-    var result = differenceInBusinessDays(
+  it('the same except given first date falls on a weekend', function () {
+    const result = differenceInBusinessDays(
       new Date(2019, 6 /* Jul */, 20),
       new Date(2019, 6 /* Jul */, 18)
     )
     assert(result === 2)
   })
 
-  it('the same except given second date falls on a weekend', function() {
-    var result = differenceInBusinessDays(
+  it('the same except given second date falls on a weekend', function () {
+    const result = differenceInBusinessDays(
       new Date(2019, 6 /* Jul */, 23),
       new Date(2019, 6 /* Jul */, 20)
     )
     assert(result === 1)
   })
 
-  it('the same except both given dates fall on a weekend', function() {
-    var result = differenceInBusinessDays(
+  it('the same except both given dates fall on a weekend', function () {
+    const result = differenceInBusinessDays(
       new Date(2019, 6 /* Jul */, 28),
       new Date(2019, 6 /* Jul */, 20)
     )
     assert(result === 5)
   })
 
-  it('returns a negative number if the time value of the first date is smaller', function() {
-    var result = differenceInBusinessDays(
+  it('returns a negative number if the time value of the first date is smaller', function () {
+    const result = differenceInBusinessDays(
       new Date(2014, 0 /* Jan */, 10),
       new Date(2014, 6 /* Jul */, 20)
     )
     assert(result === -135)
   })
 
-  it('accepts timestamps', function() {
-    var result = differenceInBusinessDays(
+  it('accepts timestamps', function () {
+    const result = differenceInBusinessDays(
       new Date(2014, 6, 18).getTime(),
       new Date(2014, 0, 10).getTime()
     )
     assert(result === 135)
   })
 
-  describe('edge cases', function() {
-    it('the difference is less than a day, but the given dates are in different calendar days', function() {
-      var result = differenceInBusinessDays(
+  describe('edge cases', function () {
+    it('the difference is less than a day, but the given dates are in different calendar days', function () {
+      const result = differenceInBusinessDays(
         new Date(2014, 8 /* Sep */, 5, 0, 0),
         new Date(2014, 8 /* Sep */, 4, 23, 59)
       )
       assert(result === 1)
     })
 
-    it('the same for the swapped dates', function() {
-      var result = differenceInBusinessDays(
+    it('the same for the swapped dates', function () {
+      const result = differenceInBusinessDays(
         new Date(2014, 8 /* Sep */, 4, 23, 59),
         new Date(2014, 8 /* Sep */, 5, 0, 0)
       )
       assert(result === -1)
     })
 
-    it('the time values of the given dates are the same', function() {
-      var result = differenceInBusinessDays(
+    it('the time values of the given dates are the same', function () {
+      const result = differenceInBusinessDays(
         new Date(2014, 8 /* Sep */, 5, 0, 0),
         new Date(2014, 8 /* Sep */, 4, 0, 0)
       )
       assert(result === 1)
     })
 
-    it('the given dates are the same', function() {
-      var result = differenceInBusinessDays(
+    it('the given dates are the same', function () {
+      const result = differenceInBusinessDays(
         new Date(2014, 8 /* Sep */, 5, 0, 0),
         new Date(2014, 8 /* Sep */, 5, 0, 0)
       )
@@ -98,41 +99,41 @@ describe('differenceInBusinessDays', function() {
     })
 
     it('does not return -0 when the given dates are the same', () => {
-      function isNegativeZero(x) {
+      function isNegativeZero(x: number) {
         return x === 0 && 1 / x < 0
       }
 
-      var result = differenceInBusinessDays(
+      const result = differenceInBusinessDays(
         new Date(2014, 8 /* Sep */, 5, 0, 0),
         new Date(2014, 8 /* Sep */, 5, 0, 0)
       )
 
-      var resultIsNegative = isNegativeZero(result)
+      const resultIsNegative = isNegativeZero(result)
       assert(resultIsNegative === false)
     })
 
-    it('returns NaN if the first date is `Invalid Date`', function() {
-      var result = differenceInBusinessDays(
+    it('returns NaN if the first date is `Invalid Date`', function () {
+      const result = differenceInBusinessDays(
         new Date(NaN),
         new Date(2017, 0 /* Jan */, 1)
       )
       assert(isNaN(result))
     })
 
-    it('returns NaN if the second date is `Invalid Date`', function() {
-      var result = differenceInBusinessDays(
+    it('returns NaN if the second date is `Invalid Date`', function () {
+      const result = differenceInBusinessDays(
         new Date(2017, 0 /* Jan */, 1),
         new Date(NaN)
       )
       assert(isNaN(result))
     })
 
-    it('returns NaN if the both dates are `Invalid Date`', function() {
-      var result = differenceInBusinessDays(new Date(NaN), new Date(NaN))
+    it('returns NaN if the both dates are `Invalid Date`', function () {
+      const result = differenceInBusinessDays(new Date(NaN), new Date(NaN))
       assert(isNaN(result))
     })
 
-    it('throws TypeError exception if passed less than 2 arguments', function() {
+    it('throws TypeError exception if passed less than 2 arguments', function () {
       assert.throws(differenceInBusinessDays.bind(null), TypeError)
       assert.throws(differenceInBusinessDays.bind(null, 1), TypeError)
     })