diff --git a/docs-site/src/examples/date_range.jsx b/docs-site/src/examples/date_range.jsx
index d0aabccc7..c7197f12a 100644
--- a/docs-site/src/examples/date_range.jsx
+++ b/docs-site/src/examples/date_range.jsx
@@ -1,6 +1,5 @@
 import React from "react";
 import DatePicker from "react-datepicker";
-import isAfter from "date-fns/isAfter";
 
 export default class DateRange extends React.Component {
   constructor(props) {
diff --git a/docs-site/src/examples/locale_without_global_variable.jsx b/docs-site/src/examples/locale_without_global_variable.jsx
index 4d265bac0..a54f1676e 100644
--- a/docs-site/src/examples/locale_without_global_variable.jsx
+++ b/docs-site/src/examples/locale_without_global_variable.jsx
@@ -1,5 +1,5 @@
 import React from "react";
-import DatePicker, { registerLocale } from "react-datepicker";
+import DatePicker from "react-datepicker";
 import fi from "date-fns/locale/fi";
 
 export default class LocaleWithoutGlobalVariable extends React.Component {
diff --git a/src/calendar.jsx b/src/calendar.jsx
index e3f105a78..705581175 100644
--- a/src/calendar.jsx
+++ b/src/calendar.jsx
@@ -213,18 +213,18 @@ export default class Calendar extends React.Component {
 
   increaseMonth = () => {
     this.setState(
-      {
-        date: addMonths(this.state.date, 1)
-      },
+      ({ date }) => ({
+        date: addMonths(date, 1)
+      }),
       () => this.handleMonthChange(this.state.date)
     );
   };
 
   decreaseMonth = () => {
     this.setState(
-      {
-        date: subMonths(this.state.date, 1)
-      },
+      ({ date }) => ({
+        date: subMonths(date, 1)
+      }),
       () => this.handleMonthChange(this.state.date)
     );
   };
@@ -269,30 +269,27 @@ export default class Calendar extends React.Component {
 
   changeYear = year => {
     this.setState(
-      {
-        date: setYear(this.state.date, year)
-      },
+      ({ date }) => ({
+        date: setYear(date, year)
+      }),
       () => this.handleYearChange(this.state.date)
     );
   };
 
   changeMonth = month => {
     this.setState(
-      {
-        date: setMonth(this.state.date, month)
-      },
+      ({ date }) => ({
+        date: setMonth(date, month)
+      }),
       () => this.handleMonthChange(this.state.date)
     );
   };
 
   changeMonthYear = monthYear => {
     this.setState(
-      {
-        date: setYear(
-          setMonth(this.state.date, getMonth(monthYear)),
-          getYear(monthYear)
-        )
-      },
+      ({ date }) => ({
+        date: setYear(setMonth(date, getMonth(monthYear)), getYear(monthYear))
+      }),
       () => this.handleMonthYearChange(this.state.date)
     );
   };
@@ -331,9 +328,9 @@ export default class Calendar extends React.Component {
 
   decreaseYear = () => {
     this.setState(
-      {
-        date: subYears(this.state.date, 1)
-      },
+      ({ date }) => ({
+        date: subYears(date, 1)
+      }),
       () => this.handleYearChange(this.state.date)
     );
   };
@@ -537,9 +534,7 @@ export default class Calendar extends React.Component {
     <div className="react-datepicker__header">
       {this.renderCurrentMonth(monthDate)}
       <div
-        className={`react-datepicker__header__dropdown react-datepicker__header__dropdown--${
-          this.props.dropdownMode
-        }`}
+        className={`react-datepicker__header__dropdown react-datepicker__header__dropdown--${this.props.dropdownMode}`}
         onFocus={this.handleDropdownFocus}
       >
         {this.renderMonthDropdown(i !== 0)}
diff --git a/src/date_utils.js b/src/date_utils.js
index b100b4b2b..aaff2a8f5 100644
--- a/src/date_utils.js
+++ b/src/date_utils.js
@@ -31,7 +31,6 @@ import max from "date-fns/max";
 import differenceInCalendarDays from "date-fns/differenceInCalendarDays";
 import differenceInCalendarMonths from "date-fns/differenceInCalendarMonths";
 import differenceInCalendarWeeks from "date-fns/differenceInCalendarWeeks";
-import setDayOfYear from "date-fns/setDayOfYear";
 import startOfDay from "date-fns/startOfDay";
 import startOfWeek from "date-fns/startOfWeek";
 import startOfMonth from "date-fns/startOfMonth";
@@ -185,7 +184,6 @@ export {
 };
 
 export function getWeek(date) {
-  let firstDayOfYear = setDayOfYear(date, 1);
   if (!isSameYear(endOfWeek(date), date)) {
     return 1;
   }