Extract differenceInMilliseconds datetime function

- Replace manual date usages with this utility function
- Remove DAY_IN_MS constant in favor of the existing one
- Update tests

app/assets/javascripts/behaviors/markdown/render_math.js

 import { deprecatedCreateFlash as flash } from '~/flash';
 import { s__, sprintf } from '~/locale';
+import { differenceInMilliseconds } from '~/lib/utils/datetime_utility';
 
 // Renders math using KaTeX in any element with the
 // `js-render-math` class
@@ -111,7 +112,7 @@ class SafeMathRenderer {
 
       // Give the browser time to reflow the svg
       waitForReflow(() => {
-        const deltaTime = Date.now() - this.startTime;
+        const deltaTime = differenceInMilliseconds(this.startTime);
         this.totalMS += deltaTime;
 
         this.renderElement();

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

@@ -216,8 +216,9 @@ export const timeFor = (time, expiredLabel) => {
   return timeago.format(time, `${timeagoLanguageCode}-remaining`).trim();
 };
 
+export const millisecondsPerDay = 1000 * 60 * 60 * 24;
+
 export const getDayDifference = (a, b) => {
-  const millisecondsPerDay = 1000 * 60 * 60 * 24;
   const date1 = Date.UTC(a.getFullYear(), a.getMonth(), a.getDate());
   const date2 = Date.UTC(b.getFullYear(), b.getMonth(), b.getDate());
 
@@ -709,7 +710,7 @@ export const dateFromParams = (year, month, day) => {
  * A utility function which computes the difference in seconds
  * between 2 dates.
  *
- * @param {Date} startDate the start sate
+ * @param {Date} startDate the start date
  * @param {Date} endDate the end date
  *
  * @return {Int} the difference in seconds
@@ -717,3 +718,18 @@ export const dateFromParams = (year, month, day) => {
 export const differenceInSeconds = (startDate, endDate) => {
   return (endDate.getTime() - startDate.getTime()) / 1000;
 };
+
+/**
+ * A utility function which computes the difference in milliseconds
+ * between 2 dates.
+ *
+ * @param {Date|Int} startDate the start date. Can be either a date object or a unix timestamp.
+ * @param {Date|Int} endDate the end date. Can be either a date object or a unix timestamp. Defaults to now.
+ *
+ * @return {Int} the difference in milliseconds
+ */
+export const differenceInMilliseconds = (startDate, endDate = Date.now()) => {
+  const startDateInMS = startDate instanceof Date ? startDate.getTime() : startDate;
+  const endDateInMS = endDate instanceof Date ? endDate.getTime() : endDate;
+  return endDateInMS - startDateInMS;
+};

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

+import { differenceInMilliseconds } from '~/lib/utils/datetime_utility';
+
 export default (fn, { interval = 2000, timeout = 60000 } = {}) => {
   const startTime = Date.now();
 
   return new Promise((resolve, reject) => {
     const stop = arg => (arg instanceof Error ? reject(arg) : resolve(arg));
     const next = () => {
-      if (timeout === 0 || Date.now() - startTime < timeout) {
+      if (timeout === 0 || differenceInMilliseconds(startTime) < timeout) {
         setTimeout(fn.bind(null, next, stop), interval);
       } else {
         reject(new Error('SIMPLE_POLL_TIMEOUT'));

ee/app/assets/javascripts/security_dashboard/components/first_class_vulnerability_chart.vue

@@ -4,10 +4,13 @@ import { GlSparklineChart } from '@gitlab/ui/dist/charts';
 import SeverityBadge from 'ee/vue_shared/security_reports/components/severity_badge.vue';
 import { s__, sprintf } from '~/locale';
 import { firstAndLastY } from '~/lib/utils/chart_utils';
-import { formatDate } from '~/lib/utils/datetime_utility';
+import {
+  formatDate,
+  differenceInMilliseconds,
+  millisecondsPerDay,
+} from '~/lib/utils/datetime_utility';
 import { formattedChangeInPercent } from '~/lib/utils/number_utils';
 import ChartButtons from './vulnerability_chart_buttons.vue';
-import { DAY_IN_MS } from '../store/modules/vulnerabilities/constants';
 import { SEVERITY_LEVELS } from '../store/constants';
 
 const ISO_DATE = 'isoDate';
@@ -74,7 +77,7 @@ export default {
   },
   computed: {
     startDate() {
-      return Date.now() - DAY_IN_MS * this.vulnerabilitiesHistoryDayRange;
+      return differenceInMilliseconds(millisecondsPerDay * this.vulnerabilitiesHistoryDayRange);
     },
     endDateCursor() {
       return Date.now();

ee/app/assets/javascripts/security_dashboard/components/vulnerability_chart.vue

@@ -9,10 +9,10 @@ import { s__, sprintf } from '~/locale';
 
 import { firstAndLastY } from '~/lib/utils/chart_utils';
 import { formattedChangeInPercent } from '~/lib/utils/number_utils';
-
+import { differenceInMilliseconds, millisecondsPerDay } from '~/lib/utils/datetime_utility';
 import ChartButtons from './vulnerability_chart_buttons.vue';
 
-import { DAY_IN_MS, DAYS } from '../store/modules/vulnerabilities/constants';
+import { DAYS } from '../store/modules/vulnerabilities/constants';
 import { SEVERITY_LEVELS } from '../store/constants';
 
 export default {
@@ -74,7 +74,7 @@ export default {
       });
     },
     startDate() {
-      return Date.now() - DAY_IN_MS * this.vulnerabilitiesHistoryDayRange;
+      return differenceInMilliseconds(millisecondsPerDay * this.vulnerabilitiesHistoryDayRange);
     },
     dateInfo() {
       const formattedStartDate = dateFormat(this.startDate, 'mmmm dS');

ee/app/assets/javascripts/security_dashboard/store/modules/vulnerabilities/constants.js

@@ -8,8 +8,6 @@ export const INFO = 'info';
 export const UNKNOWN = 'unknown';
 export const SEVERITIES = [CRITICAL, HIGH, MEDIUM, LOW, INFO, UNKNOWN];
 
-export const DAY_IN_MS = 1000 * 60 * 60 * 24;
-
 export const DAYS = {
   THIRTY: 30,
   SIXTY: 60,

ee/spec/frontend/geo_nodes/components/geo_node_last_updated_spec.js

 import { shallowMount } from '@vue/test-utils';
 import { GlPopover, GlLink, GlIcon } from '@gitlab/ui';
-
 import GeoNodeLastUpdated from 'ee/geo_nodes/components/geo_node_last_updated.vue';
 import {
   HELP_NODE_HEALTH_URL,
   GEO_TROUBLESHOOTING_URL,
   STATUS_DELAY_THRESHOLD_MS,
 } from 'ee/geo_nodes/constants';
+import { differenceInMilliseconds } from '~/lib/utils/datetime_utility';
 
 describe('GeoNodeLastUpdated', () => {
   let wrapper;
 
-  const staleStatusTime = new Date(Date.now() - STATUS_DELAY_THRESHOLD_MS).getTime();
+  const staleStatusTime = differenceInMilliseconds(STATUS_DELAY_THRESHOLD_MS);
   const nonStaleStatusTime = new Date().getTime();
 
   const defaultProps = {

ee/spec/frontend/vue_shared/dashboards/components/time_ago_spec.js

 import { shallowMount } from '@vue/test-utils';
 import TimeAgo from 'ee/vue_shared/dashboards/components/time_ago.vue';
+import { differenceInMilliseconds } from '~/lib/utils/datetime_utility';
 
 describe('time ago component', () => {
   let wrapper;
@@ -7,7 +8,7 @@ describe('time ago component', () => {
   beforeEach(() => {
     wrapper = shallowMount(TimeAgo, {
       propsData: {
-        time: new Date(Date.now() - 86400000).toISOString(),
+        time: new Date(differenceInMilliseconds(86400000)).toISOString(),
         tooltipText: 'Finished',
       },
     });

ee/spec/frontend/vue_shared/dashboards/mock_data.js

 import { TEST_HOST } from 'spec/test_constants';
+import { differenceInMilliseconds } from '~/lib/utils/datetime_utility';
 
 const AVATAR_URL = `${TEST_HOST}/dummy.jpg`;
 
@@ -21,7 +22,7 @@ export const mockHeaders = {
 export function mockPipelineData(
   status = 'success',
   id = 1,
-  finishedTimeStamp = new Date(Date.now() - 86400000).toISOString(),
+  finishedTimeStamp = new Date(differenceInMilliseconds(86400000)).toISOString(),
   isTag = false,
 ) {
   return {

spec/frontend/environments/environment_item_spec.js

@@ -3,7 +3,7 @@ import { format } from 'timeago.js';
 import EnvironmentItem from '~/environments/components/environment_item.vue';
 import PinComponent from '~/environments/components/environment_pin.vue';
 import DeleteComponent from '~/environments/components/environment_delete.vue';
-
+import { differenceInMilliseconds } from '~/lib/utils/datetime_utility';
 import { environment, folder, tableData } from './mock_data';
 
 describe('Environment item', () => {
@@ -135,7 +135,7 @@ describe('Environment item', () => {
         });
 
         describe('in the past', () => {
-          const pastDate = new Date(Date.now() - 100000);
+          const pastDate = new Date(differenceInMilliseconds(100000));
           beforeEach(() => {
             factory({
               propsData: {

spec/frontend/lib/utils/datetime_utility_spec.js

@@ -653,3 +653,17 @@ describe('differenceInSeconds', () => {
     expect(datetimeUtility.differenceInSeconds(startDate, endDate)).toBe(expected);
   });
 });
+
+describe('differenceInMilliseconds', () => {
+  const startDateTime = new Date('2019-07-17T00:00:00.000Z');
+
+  it.each`
+    startDate                               | endDate                                           | expected
+    ${startDateTime.getTime()}              | ${new Date('2019-07-17T00:00:00.000Z')}           | ${0}
+    ${startDateTime}                        | ${new Date('2019-07-17T12:00:00.000Z').getTime()} | ${43200000}
+    ${startDateTime}                        | ${new Date('2019-07-18T00:00:00.000Z').getTime()} | ${86400000}
+    ${new Date('2019-07-18T00:00:00.000Z')} | ${startDateTime.getTime()}                        | ${-86400000}
+  `('returns $expected for $endDate - $startDate', ({ startDate, endDate, expected }) => {
+    expect(datetimeUtility.differenceInMilliseconds(startDate, endDate)).toBe(expected);
+  });
+});

spec/frontend/snippets/components/snippet_header_spec.js

@@ -5,6 +5,7 @@ import { Blob, BinaryBlob } from 'jest/blob/components/mock_data';
 import waitForPromises from 'helpers/wait_for_promises';
 import DeleteSnippetMutation from '~/snippets/mutations/deleteSnippet.mutation.graphql';
 import SnippetHeader from '~/snippets/components/snippet_header.vue';
+import { differenceInMilliseconds } from '~/lib/utils/datetime_utility';
 
 describe('Snippet header component', () => {
   let wrapper;
@@ -67,7 +68,7 @@ describe('Snippet header component', () => {
         name: 'Thor Odinson',
       },
       blobs: [Blob],
-      createdAt: new Date(Date.now() - 32 * 24 * 3600 * 1000).toISOString(),
+      createdAt: new Date(differenceInMilliseconds(32 * 24 * 3600 * 1000)).toISOString(),
     };
 
     mutationVariables = {