Fix getDateInPast method

- Avoid mutating the original date reference

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

@@ -555,5 +555,9 @@ export const calculateRemainingMilliseconds = endDate => {
  * @param {number} daysInPast number of days that are subtracted from a given date
  * @returns {String} Date string in ISO format
  */
-export const getDateInPast = (date, daysInPast) =>
-  new Date(date.setTime(date.getTime() - daysInPast * 24 * 60 * 60 * 1000)).toISOString();
+export const getDateInPast = (date, daysInPast) => {
+  const dateClone = newDate(date);
+  return new Date(
+    dateClone.setTime(dateClone.getTime() - daysInPast * 24 * 60 * 60 * 1000),
+  ).toISOString();
+};

spec/frontend/lib/utils/datetime_utility_spec.js

@@ -428,11 +428,16 @@ describe('newDate', () => {
 });
 
 describe('getDateInPast', () => {
+  const date = new Date(1563235200000); // 2019-07-16T00:00:00.000Z;
+  const daysInPast = 90;
+
   it('returns the correct date in the past', () => {
-    const date = new Date(1563235200000); // 2019-07-16T00:00:00.00Z
-    const daysInPast = 90;
     const dateInPast = datetimeUtility.getDateInPast(date, daysInPast);
-
     expect(dateInPast).toBe('2019-04-17T00:00:00.000Z');
   });
+
+  it('does not modifiy the original date', () => {
+    datetimeUtility.getDateInPast(date, daysInPast);
+    expect(date).toStrictEqual(new Date(1563235200000));
+  });
 });