Add getDateInFuture util method

This is a util method within the datetime_utility which
is used to get a date in the future. It accepts a date
param along with a param for the number of days which is
used for determining the future date.

app/assets/javascripts/lib/utils/datetime_utility.js

@@ -555,6 +555,16 @@ export const calculateRemainingMilliseconds = endDate => {
 export const getDateInPast = (date, daysInPast) =>
   new Date(newDate(date).setDate(date.getDate() - daysInPast));
 
+/**
+ * Adds a given number of days to a given date and returns the new date.
+ *
+ * @param {Date} date the date that we will add days to
+ * @param {Number} daysInFuture number of days that are added to a given date
+ * @returns {Date} Date in future as Date object
+ */
+export const getDateInFuture = (date, daysInFuture) =>
+  new Date(newDate(date).setDate(date.getDate() + daysInFuture));
+
 /*
  * Appending T00:00:00 makes JS assume local time and prevents it from shifting the date
  * to match the user's time zone. We want to display the date in server time for now, to

changelogs/unreleased/add-get-date-in-future-util.yml

+---
+title: Add getDateInFuture util method
+merge_request: 22671
+author:
+type: added

spec/frontend/lib/utils/datetime_utility_spec.js

@@ -455,6 +455,23 @@ describe('getDateInPast', () => {
   });
 });
 
+describe('getDateInFuture', () => {
+  const date = new Date('2019-07-16T00:00:00.000Z');
+  const daysInFuture = 90;
+
+  it('returns the correct date in the future', () => {
+    const dateInFuture = datetimeUtility.getDateInFuture(date, daysInFuture);
+    const expectedDateInFuture = new Date('2019-10-14T00:00:00.000Z');
+
+    expect(dateInFuture).toStrictEqual(expectedDateInFuture);
+  });
+
+  it('does not modifiy the original date', () => {
+    datetimeUtility.getDateInFuture(date, daysInFuture);
+    expect(date).toStrictEqual(new Date('2019-07-16T00:00:00.000Z'));
+  });
+});
+
 describe('getDatesInRange', () => {
   it('returns an empty array if 1st or 2nd argument is not a Date object', () => {
     const d1 = new Date('2019-01-01');