diff --git a/src/Plugin/Field/FieldWidget/DateRecurModularAlphaWidget.php b/src/Plugin/Field/FieldWidget/DateRecurModularAlphaWidget.php
index 191e5ec..76c0004 100644
--- a/src/Plugin/Field/FieldWidget/DateRecurModularAlphaWidget.php
+++ b/src/Plugin/Field/FieldWidget/DateRecurModularAlphaWidget.php
@@ -268,6 +268,9 @@ class DateRecurModularAlphaWidget extends DateRecurModularWidgetBase {
     /** @var string|null $timeZone */
     $timeZone = $form_state->getValue(array_merge($element['#parents'], ['time_zone']));
 
+    if ($start instanceof DrupalDateTime && $end instanceof DrupalDateTime && ($start->getPhpDateTime() > $end->getPhpDateTime())) {
+      $form_state->setError($element['end'], \t('End time must be after start time.'));
+    }
     if ($start && !$timeZone) {
       $form_state->setError($element['start'], \t('Time zone must be set if start date is set.'));
     }
diff --git a/tests/src/Functional/DateRecurModularAlphaTest.php b/tests/src/Functional/DateRecurModularAlphaTest.php
new file mode 100644
index 0000000..a7fb374
--- /dev/null
+++ b/tests/src/Functional/DateRecurModularAlphaTest.php
@@ -0,0 +1,486 @@
+<?php
+
+declare(strict_types = 1);
+
+namespace Drupal\Tests\date_recur_modular\Functional;
+
+use Drupal\date_recur_entity_test\Entity\DrEntityTest;
+use Drupal\FunctionalJavascriptTests\WebDriverTestBase;
+
+/**
+ * Tests Alpha Widget.
+ *
+ * @group date_recur_modular_widget
+ * @coversDefaultClass \Drupal\date_recur_modular\Plugin\Field\FieldWidget\DateRecurModularAlphaWidget
+ */
+class DateRecurModularAlphaTest extends WebDriverTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = [
+    'date_recur_modular',
+    'date_recur_entity_test',
+    'entity_test',
+    'datetime',
+    'datetime_range',
+    'date_recur',
+    'field',
+    'user',
+    'system',
+  ];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    // @todo replace in 8.8: See 2835616.
+    $display = \entity_get_form_display('dr_entity_test', 'dr_entity_test', 'default');
+    $component = $display->getComponent('dr');
+    $component['region'] = 'content';
+    $component['type'] = 'date_recur_modular_alpha';
+    $component['settings'] = [];
+    $display->setComponent('dr', $component);
+    $display->save();
+
+    $user = $this->drupalCreateUser(['administer entity_test content']);
+    $user->timezone = 'Asia/Singapore';
+    $user->save();
+    $this->drupalLogin($user);
+  }
+
+  /**
+   * Tests field widget input is converted to appropriate database values.
+   *
+   * @param array $values
+   *   Array of form fields to submit.
+   * @param array $expected
+   *   Array of expected field normalized values.
+   * @param bool $clickAllDay
+   *   Whether to click the all day toggle.
+   *
+   * @dataProvider providerTestWidget
+   */
+  public function testWidget(array $values, array $expected, $clickAllDay = FALSE): void {
+    $entity = DrEntityTest::create();
+    $entity->save();
+    $this->drupalGet($entity->toUrl('edit-form'));
+
+    if ($clickAllDay) {
+      $this->getSession()->getPage()->find('css', '.parts--is-all-day .form-radios> *:nth-child(1) label')->click();
+    }
+
+    $this->drupalPostForm(NULL, $values, 'Save');
+    $this->assertSession()->pageTextContains('has been updated.');
+
+    $entity = DrEntityTest::load($entity->id());
+    $this->assertEquals($expected, $entity->dr[0]->getValue());
+  }
+
+  /**
+   * Data provider for testWidget()
+   *
+   * @return array
+   *   Data for testing.
+   */
+  public function providerTestWidget(): array {
+    $data = [];
+
+    $data['once'] = [
+      [
+        'dr[0][mode]' => 'once',
+        'dr[0][day_start]' => '04/14/2015',
+        'dr[0][times][time_start]' => '09:00:00am',
+        'dr[0][times][time_end]' => '05:00:00pm',
+      ],
+      [
+        'value' => '2015-04-14T01:00:00',
+        'end_value' => '2015-04-14T09:00:00',
+        'rrule' => NULL,
+        'infinite' => FALSE,
+        'timezone' => 'Asia/Singapore',
+      ],
+    ];
+
+    $data['multi'] = [
+      [
+        'dr[0][mode]' => 'multiday',
+        'dr[0][daily_count]' => 3,
+        'dr[0][day_start]' => '04/14/2015',
+        'dr[0][times][time_start]' => '09:00:00am',
+        'dr[0][times][time_end]' => '5:00:00pm',
+      ],
+      [
+        'value' => '2015-04-14T01:00:00',
+        'end_value' => '2015-04-14T09:00:00',
+        'rrule' => 'FREQ=DAILY;INTERVAL=1;COUNT=3',
+        'infinite' => FALSE,
+        'timezone' => 'Asia/Singapore',
+      ],
+    ];
+
+    $data['weekly'] = [
+      [
+        'dr[0][mode]' => 'weekly',
+        'dr[0][weekdays][MO]' => TRUE,
+        'dr[0][weekdays][WE]' => TRUE,
+        'dr[0][weekdays][FR]' => TRUE,
+        'dr[0][day_start]' => '04/14/2015',
+        'dr[0][times][time_start]' => '09:00:00am',
+        'dr[0][times][time_end]' => '05:00:00pm',
+      ],
+      [
+        'value' => '2015-04-14T01:00:00',
+        'end_value' => '2015-04-14T09:00:00',
+        'rrule' => 'FREQ=WEEKLY;INTERVAL=1;BYDAY=MO,WE,FR',
+        'infinite' => TRUE,
+        'timezone' => 'Asia/Singapore',
+      ],
+    ];
+
+    $data['fortnightly'] = [
+      [
+        'dr[0][mode]' => 'fortnightly',
+        'dr[0][weekdays][MO]' => TRUE,
+        'dr[0][weekdays][WE]' => TRUE,
+        'dr[0][weekdays][FR]' => TRUE,
+        'dr[0][day_start]' => '04/14/2015',
+        'dr[0][times][time_start]' => '09:00:00am',
+        'dr[0][times][time_end]' => '05:00:00pm',
+      ],
+      [
+        'value' => '2015-04-14T01:00:00',
+        'end_value' => '2015-04-14T09:00:00',
+        'rrule' => 'FREQ=WEEKLY;INTERVAL=2;BYDAY=MO,WE,FR',
+        'infinite' => TRUE,
+        'timezone' => 'Asia/Singapore',
+      ],
+    ];
+
+    $data['allday'] = [
+      [
+        'dr[0][mode]' => 'once',
+        'dr[0][day_start]' => '04/14/2015',
+      ],
+      [
+        'value' => '2015-04-13T16:00:00',
+        'end_value' => '2015-04-14T15:59:59',
+        'rrule' => NULL,
+        'infinite' => FALSE,
+        'timezone' => 'Asia/Singapore',
+      ],
+      TRUE,
+    ];
+
+    // First Friday of the month.
+    $data['monthly 1 ordinal 1 weekday'] = [
+      [
+        'dr[0][mode]' => 'monthly',
+        'dr[0][day_start]' => '04/14/2015',
+        'dr[0][times][time_start]' => '09:00:00am',
+        'dr[0][times][time_end]' => '05:00:00pm',
+        // Set weekday first, ordinals will appear after it.
+        'dr[0][weekdays][FR]' => TRUE,
+        'dr[0][ordinals][1]' => TRUE,
+      ],
+      [
+        'value' => '2015-04-14T01:00:00',
+        'end_value' => '2015-04-14T09:00:00',
+        'rrule' => 'FREQ=MONTHLY;INTERVAL=1;BYDAY=FR;BYSETPOS=1',
+        'infinite' => TRUE,
+        'timezone' => 'Asia/Singapore',
+      ],
+    ];
+
+    // First Thursday and Friday of the month.
+    $data['monthly 1 ordinal 2 weekday'] = [
+      [
+        'dr[0][mode]' => 'monthly',
+        'dr[0][day_start]' => '04/14/2015',
+        'dr[0][times][time_start]' => '09:00:00am',
+        'dr[0][times][time_end]' => '05:00:00pm',
+        'dr[0][weekdays][TH]' => TRUE,
+        'dr[0][weekdays][FR]' => TRUE,
+        'dr[0][ordinals][1]' => TRUE,
+      ],
+      [
+        'value' => '2015-04-14T01:00:00',
+        'end_value' => '2015-04-14T09:00:00',
+        'rrule' => 'FREQ=MONTHLY;INTERVAL=1;BYDAY=TH,FR;BYSETPOS=1,2',
+        'infinite' => TRUE,
+        'timezone' => 'Asia/Singapore',
+      ],
+    ];
+
+    // First and second Friday of the month.
+    $data['monthly 1,2 ordinal 1 weekday'] = [
+      [
+        'dr[0][mode]' => 'monthly',
+        'dr[0][day_start]' => '04/14/2015',
+        'dr[0][times][time_start]' => '09:00:00am',
+        'dr[0][times][time_end]' => '05:00:00pm',
+        'dr[0][weekdays][FR]' => TRUE,
+        'dr[0][ordinals][1]' => TRUE,
+        'dr[0][ordinals][2]' => TRUE,
+      ],
+      [
+        'value' => '2015-04-14T01:00:00',
+        'end_value' => '2015-04-14T09:00:00',
+        'rrule' => 'FREQ=MONTHLY;INTERVAL=1;BYDAY=FR;BYSETPOS=1,2',
+        'infinite' => TRUE,
+        'timezone' => 'Asia/Singapore',
+      ],
+    ];
+
+    // First and second Thursday and Friday of the month.
+    $data['monthly 1,2 ordinal 2 weekday'] = [
+      [
+        'dr[0][mode]' => 'monthly',
+        'dr[0][day_start]' => '04/14/2015',
+        'dr[0][times][time_start]' => '09:00:00am',
+        'dr[0][times][time_end]' => '05:00:00pm',
+        'dr[0][weekdays][TH]' => TRUE,
+        'dr[0][weekdays][FR]' => TRUE,
+        'dr[0][ordinals][1]' => TRUE,
+        'dr[0][ordinals][2]' => TRUE,
+      ],
+      [
+        'value' => '2015-04-14T01:00:00',
+        'end_value' => '2015-04-14T09:00:00',
+        'rrule' => 'FREQ=MONTHLY;INTERVAL=1;BYDAY=TH,FR;BYSETPOS=1,2,3,4',
+        'infinite' => TRUE,
+        'timezone' => 'Asia/Singapore',
+      ],
+    ];
+
+    // Last Thursday of the month.
+    $data['monthly -1 ordinal 1 weekday'] = [
+      [
+        'dr[0][mode]' => 'monthly',
+        'dr[0][day_start]' => '04/14/2015',
+        'dr[0][times][time_start]' => '09:00:00am',
+        'dr[0][times][time_end]' => '05:00:00pm',
+        'dr[0][weekdays][TH]' => TRUE,
+        'dr[0][ordinals][-1]' => TRUE,
+      ],
+      [
+        'value' => '2015-04-14T01:00:00',
+        'end_value' => '2015-04-14T09:00:00',
+        'rrule' => 'FREQ=MONTHLY;INTERVAL=1;BYDAY=TH;BYSETPOS=-1',
+        'infinite' => TRUE,
+        'timezone' => 'Asia/Singapore',
+      ],
+    ];
+
+    // Last Thursday and Friday of the month.
+    $data['monthly -1 ordinal 2 weekday'] = [
+      [
+        'dr[0][mode]' => 'monthly',
+        'dr[0][day_start]' => '04/14/2015',
+        'dr[0][times][time_start]' => '09:00:00am',
+        'dr[0][times][time_end]' => '05:00:00pm',
+        'dr[0][weekdays][TH]' => TRUE,
+        'dr[0][weekdays][FR]' => TRUE,
+        'dr[0][ordinals][-1]' => TRUE,
+      ],
+      [
+        'value' => '2015-04-14T01:00:00',
+        'end_value' => '2015-04-14T09:00:00',
+        'rrule' => 'FREQ=MONTHLY;INTERVAL=1;BYDAY=TH,FR;BYSETPOS=-2,-1',
+        'infinite' => TRUE,
+        'timezone' => 'Asia/Singapore',
+      ],
+    ];
+
+    // Second to last Thursday of the month.
+    $data['monthly -2 ordinal 1 weekday'] = [
+      [
+        'dr[0][mode]' => 'monthly',
+        'dr[0][day_start]' => '04/14/2015',
+        'dr[0][times][time_start]' => '09:00:00am',
+        'dr[0][times][time_end]' => '05:00:00pm',
+        'dr[0][weekdays][TH]' => TRUE,
+        'dr[0][ordinals][-2]' => TRUE,
+      ],
+      [
+        'value' => '2015-04-14T01:00:00',
+        'end_value' => '2015-04-14T09:00:00',
+        'rrule' => 'FREQ=MONTHLY;INTERVAL=1;BYDAY=TH;BYSETPOS=-2',
+        'infinite' => TRUE,
+        'timezone' => 'Asia/Singapore',
+      ],
+    ];
+
+    // Second to last Thursday and Friday of the month.
+    $data['monthly -4,-3 ordinal 2 weekday'] = [
+      [
+        'dr[0][mode]' => 'monthly',
+        'dr[0][day_start]' => '04/14/2015',
+        'dr[0][times][time_start]' => '09:00:00am',
+        'dr[0][times][time_end]' => '05:00:00pm',
+        'dr[0][weekdays][TH]' => TRUE,
+        'dr[0][weekdays][FR]' => TRUE,
+        'dr[0][ordinals][-2]' => TRUE,
+      ],
+      [
+        'value' => '2015-04-14T01:00:00',
+        'end_value' => '2015-04-14T09:00:00',
+        'rrule' => 'FREQ=MONTHLY;INTERVAL=1;BYDAY=TH,FR;BYSETPOS=-4,-3',
+        'infinite' => TRUE,
+        'timezone' => 'Asia/Singapore',
+      ],
+    ];
+
+    // Last and Second to last Thursday and Friday of the month.
+    $data['monthly -4,-3-2,-1 ordinal 2 weekday'] = [
+      [
+        'dr[0][mode]' => 'monthly',
+        'dr[0][day_start]' => '04/14/2015',
+        'dr[0][times][time_start]' => '09:00:00am',
+        'dr[0][times][time_end]' => '05:00:00pm',
+        'dr[0][weekdays][TH]' => TRUE,
+        'dr[0][weekdays][FR]' => TRUE,
+        'dr[0][ordinals][-1]' => TRUE,
+        'dr[0][ordinals][-2]' => TRUE,
+      ],
+      [
+        'value' => '2015-04-14T01:00:00',
+        'end_value' => '2015-04-14T09:00:00',
+        'rrule' => 'FREQ=MONTHLY;INTERVAL=1;BYDAY=TH,FR;BYSETPOS=-4,-3,-2,-1',
+        'infinite' => TRUE,
+        'timezone' => 'Asia/Singapore',
+      ],
+    ];
+
+    // Combination second and second to last Thursday and Friday of the month.
+    $data['monthly -4,-3,3,4 ordinal 2 weekday'] = [
+      [
+        'dr[0][mode]' => 'monthly',
+        'dr[0][day_start]' => '04/14/2015',
+        'dr[0][times][time_start]' => '09:00:00am',
+        'dr[0][times][time_end]' => '05:00:00pm',
+        'dr[0][weekdays][TH]' => TRUE,
+        'dr[0][weekdays][FR]' => TRUE,
+        'dr[0][ordinals][2]' => TRUE,
+        'dr[0][ordinals][-2]' => TRUE,
+      ],
+      [
+        'value' => '2015-04-14T01:00:00',
+        'end_value' => '2015-04-14T09:00:00',
+        'rrule' => 'FREQ=MONTHLY;INTERVAL=1;BYDAY=TH,FR;BYSETPOS=-4,-3,3,4',
+        'infinite' => TRUE,
+        'timezone' => 'Asia/Singapore',
+      ],
+    ];
+
+    return $data;
+  }
+
+  /**
+   * Tests times fields end before start.
+   */
+  public function testTimesEndBeforeStart(): void {
+    $entity = DrEntityTest::create();
+    $entity->save();
+
+    $edit = [
+      'dr[0][mode]' => 'weekly',
+      'dr[0][day_start]' => '04/14/2015',
+      'dr[0][times][time_start]' => '09:00:00am',
+      'dr[0][times][time_end]' => '08:00:00am',
+      'dr[0][weekdays][MO]' => TRUE,
+    ];
+    $this->drupalPostForm($entity->toUrl('edit-form'), $edit, 'Save');
+    $this->assertSession()->pageTextContains('End time must be after start time.');
+  }
+
+  /**
+   * Tests times fields end same as start.
+   */
+  public function testTimesEndEqualStart(): void {
+    $entity = DrEntityTest::create();
+    $entity->save();
+
+    $edit = [
+      'dr[0][mode]' => 'weekly',
+      'dr[0][day_start]' => '04/14/2015',
+      'dr[0][times][time_start]' => '09:00:00am',
+      'dr[0][times][time_end]' => '09:00:00am',
+      'dr[0][weekdays][MO]' => TRUE,
+    ];
+    $this->drupalPostForm($entity->toUrl('edit-form'), $edit, 'Save');
+    $this->assertSession()->pageTextContains('has been updated.');
+  }
+
+  /**
+   * Tests times fields end after start.
+   */
+  public function testTimesEndAfterStart(): void {
+    $entity = DrEntityTest::create();
+    $entity->save();
+
+    $edit = [
+      'dr[0][mode]' => 'weekly',
+      'dr[0][day_start]' => '04/14/2015',
+      'dr[0][times][time_start]' => '09:00:00am',
+      'dr[0][times][time_end]' => '10:00:00am',
+      'dr[0][weekdays][MO]' => TRUE,
+    ];
+    $this->drupalPostForm($entity->toUrl('edit-form'), $edit, 'Save');
+    $this->assertSession()->pageTextContains('has been updated.');
+  }
+
+  /**
+   * Tests times fields end not set.
+   */
+  public function testTimesStartNotSet(): void {
+    $entity = DrEntityTest::create();
+    $entity->save();
+
+    $edit = [
+      'dr[0][mode]' => 'weekly',
+      'dr[0][day_start]' => '04/14/2015',
+      'dr[0][times][time_end]' => '09:00:00am',
+      'dr[0][weekdays][MO]' => TRUE,
+    ];
+    $this->drupalPostForm($entity->toUrl('edit-form'), $edit, 'Save');
+    $this->assertSession()->pageTextContains('Invalid start time.');
+  }
+
+  /**
+   * Tests times fields end not set.
+   */
+  public function testTimesEndNotSet(): void {
+    $entity = DrEntityTest::create();
+    $entity->save();
+
+    $edit = [
+      'dr[0][mode]' => 'weekly',
+      'dr[0][day_start]' => '04/14/2015',
+      'dr[0][times][time_start]' => '09:00:00am',
+      'dr[0][weekdays][MO]' => TRUE,
+    ];
+    $this->drupalPostForm($entity->toUrl('edit-form'), $edit, 'Save');
+    $this->assertSession()->pageTextContains('Invalid end time.');
+  }
+
+  /**
+   * Tests times fields end not set.
+   */
+  public function testTimesDayNotSet(): void {
+    $entity = DrEntityTest::create();
+    $entity->save();
+
+    $edit = [
+      'dr[0][mode]' => 'weekly',
+      'dr[0][times][time_start]' => '09:00:00am',
+      'dr[0][times][time_end]' => '09:00:00am',
+      'dr[0][weekdays][MO]' => TRUE,
+    ];
+    $this->drupalPostForm($entity->toUrl('edit-form'), $edit, 'Save');
+    $this->assertSession()->pageTextContains('Invalid start day.');
+  }
+
+}