fix(docs-infra): correctly display event dates on all timezones (#41053)

Previously, the event dates displayed on the angular.io "Events" page
(`/events`) was off by one day on timezones with a negative offset from
UTC. See
https://github.com/angular/angular/pull/41050#issuecomment-788958888.

This commit fixes it by using the `getUTC*` methods of the `Date` object
to extract the date info, which are not affected by the user's timezone.

PR Close #41053

aio/package.json

@@ -161,6 +161,7 @@
     "rimraf": "^2.6.1",
     "semver": "^5.3.0",
     "shelljs": "^0.8.4",
+    "timezone-mock": "^1.1.3",
     "tree-kill": "^1.1.0",
     "ts-node": "^8.4.1",
     "tslint": "~6.1.0",

aio/src/app/custom-elements/events/events.component.spec.ts

@@ -1,5 +1,6 @@
 import { Injector } from '@angular/core';
 import { Subject } from 'rxjs';
+import * as tzMock from 'timezone-mock';
 import { Duration, Event, EventsComponent } from './events.component';
 import { EventsService } from './events.service';
 
@@ -132,6 +133,20 @@ describe('EventsComponent', () => {
   });
 
   describe('getEventDates()', () => {
+    // Test on different timezones to ensure that event dates are processed correctly regardless of
+    // the user's local time.
+    const timezones: tzMock.TimeZone[] = [
+      'Australia/Adelaide',  // UTC+9.5/10.5
+      'Brazil/East',         // UTC-3
+      'UTC',                 // UTC
+    ];
+
+    for (const tz of timezones) {
+      describe(`on timezone ${tz}`, () => {
+        // NOTE: `timezone-mock` does not work correctly if used together with Jasmine's mock clock.
+        beforeEach(() => tzMock.register(tz));
+        afterEach(() => tzMock.unregister());
+
         describe('(without workshops)', () => {
           it('should correctly format the main event date', () => {
             const testEvent = createMockEvent('Test', {start: '2020-06-20', end: '2020-06-20'});
@@ -147,6 +162,11 @@ describe('EventsComponent', () => {
             const testEvent = createMockEvent('Test', {start: '2019-10-30', end: '2019-11-01'});
             expect(component.getEventDates(testEvent)).toBe('October 30 - November 1, 2019');
           });
+
+          it('should correctly format event dates at the beginning/end of the year', () => {
+            const testEvent = createMockEvent('Test', {start: '2021-01-01', end: '2021-12-31'});
+            expect(component.getEventDates(testEvent)).toBe('January 1 - December 31, 2021');
+          });
         });
 
         describe('(with workshops)', () => {
@@ -213,6 +233,8 @@ describe('EventsComponent', () => {
           });
         });
       });
+    }
+  });
 
   // Helpers
   class TestEventsService {

aio/src/app/custom-elements/events/events.component.ts

@@ -72,7 +72,7 @@ export class EventsComponent implements OnInit {
       // If no work shop date create conference date string
       dateString = processDate(event.date);
     }
-    dateString = `${dateString}, ${new Date(event.date.end).getFullYear()}`;
+    dateString = `${dateString}, ${new Date(event.date.end).getUTCFullYear()}`;
     return dateString;
   }
 }
@@ -83,14 +83,14 @@ function processDate(dates: Duration) {
   const endDate = new Date(dates.end);
 
   // Create a date string in the start like January 31
-  let processedDate = `${MONTHS[startDate.getMonth()]} ${startDate.getDate()}`;
+  let processedDate = `${MONTHS[startDate.getUTCMonth()]} ${startDate.getUTCDate()}`;
 
   // If they are in different months add the string '- February 2' Making the final string January 31 - February 2
-  if (startDate.getMonth() !== endDate.getMonth()) {
-    processedDate = `${processedDate} - ${MONTHS[endDate.getMonth()]} ${endDate.getDate()}`;
-  } else if (startDate.getDate() !== endDate.getDate()) {
+  if (startDate.getUTCMonth() !== endDate.getUTCMonth()) {
+    processedDate = `${processedDate} - ${MONTHS[endDate.getUTCMonth()]} ${endDate.getUTCDate()}`;
+  } else if (startDate.getUTCDate() !== endDate.getUTCDate()) {
     // If not add - date eg it will make // January 30-31
-    processedDate = `${processedDate}-${endDate.getDate()}`;
+    processedDate = `${processedDate}-${endDate.getUTCDate()}`;
   }
 
   return processedDate;

aio/yarn.lock

@@ -13168,6 +13168,11 @@ timers-ext@^0.1.5:
     es5-ext "~0.10.46"
     next-tick "1"
 
+timezone-mock@^1.1.3:
+  version "1.1.3"
+  resolved "https://registry.yarnpkg.com/timezone-mock/-/timezone-mock-1.1.3.tgz#f5ca25befec2cdd2d64c07ee7a5293275c06b97e"
+  integrity sha512-r+fz9M1tflNkF6mJK0DwmlFyNWeSCXxZ68pu2Bv+DddIHK/czOZwnasEql17VfHqP/OcZRuPq/qi6wm7eQmQow==
+
 timsort@^0.3.0:
   version "0.3.0"
   resolved "https://registry.yarnpkg.com/timsort/-/timsort-0.3.0.tgz#405411a8e7e6339fe64db9a234de11dc31e02bd4"