Add calculateRemainingMilliseconds() helper function

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

@@ -473,3 +473,15 @@ export const stringifyTime = timeObject => {
  */
 export const abbreviateTime = timeStr =>
   timeStr.split(' ').filter(unitStr => unitStr.charAt(0) !== '0')[0];
+
+/**
+ * Calculates the milliseconds between now and a given date string.
+ * The result cannot become negative.
+ *
+ * @param endDate date string that the time difference is calculated for
+ * @return {number} number of milliseconds remaining until the given date
+ */
+export const calculateRemainingMilliseconds = endDate => {
+  const remainingMilliseconds = new Date(endDate).getTime() - Date.now();
+  return Math.max(remainingMilliseconds, 0);
+};

spec/javascripts/lib/utils/datetime_utility_spec.js

@@ -352,3 +352,21 @@ describe('prettyTime methods', () => {
     });
   });
 });
+
+describe('calculateRemainingMilliseconds', () => {
+  beforeEach(() => {
+    spyOn(Date, 'now').and.callFake(() => new Date('2063-04-04T00:42:00Z').getTime());
+  });
+
+  it('calculates the remaining time for a given end date', () => {
+    const milliseconds = datetimeUtility.calculateRemainingMilliseconds('2063-04-04T01:44:03Z');
+
+    expect(milliseconds).toBe(3723000);
+  });
+
+  it('returns 0 if the end date has passed', () => {
+    const milliseconds = datetimeUtility.calculateRemainingMilliseconds('2063-04-03T00:00:00Z');
+
+    expect(milliseconds).toBe(0);
+  });
+});