diff --git a/mod/forum/tests/externallib_test.php b/mod/forum/tests/externallib_test.php
index 786f350c8cc..8a1df829d43 100644
--- a/mod/forum/tests/externallib_test.php
+++ b/mod/forum/tests/externallib_test.php
@@ -35,7 +35,7 @@ require_once($CFG->dirroot . '/mod/forum/lib.php');
  * @copyright  2012 Mark Nelson <markn@moodle.com>
  * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
  */
-class externallib_test extends externallib_advanced_testcase {
+final class externallib_test extends externallib_advanced_testcase {
 
     /**
      * Tests set up
@@ -1056,6 +1056,8 @@ class externallib_test extends externallib_advanced_testcase {
 
         $this->resetAfterTest(true);
 
+        $clock = $this->mock_clock_with_frozen();
+
         // Set the CFG variable to allow track forums.
         $CFG->forum_trackreadposts = true;
 
@@ -1105,7 +1107,7 @@ class externallib_test extends externallib_advanced_testcase {
         $record->userid = $user1->id;
         $record->forum = $forum1->id;
         $discussion1 = self::getDataGenerator()->get_plugin_generator('mod_forum')->create_discussion($record);
-        sleep(1);
+        $clock->bump();
 
         // Add three replies to the discussion 1 from different users.
         $record = new \stdClass();
@@ -1113,16 +1115,16 @@ class externallib_test extends externallib_advanced_testcase {
         $record->parent = $discussion1->firstpost;
         $record->userid = $user2->id;
         $discussion1reply1 = self::getDataGenerator()->get_plugin_generator('mod_forum')->create_post($record);
-        sleep(1);
+        $clock->bump();
 
         $record->parent = $discussion1reply1->id;
         $record->userid = $user3->id;
         $discussion1reply2 = self::getDataGenerator()->get_plugin_generator('mod_forum')->create_post($record);
-        sleep(1);
+        $clock->bump();
 
         $record->userid = $user4->id;
         $discussion1reply3 = self::getDataGenerator()->get_plugin_generator('mod_forum')->create_post($record);
-        sleep(1);
+        $clock->bump();
 
         // Create discussion2.
         $record2 = new \stdClass();
@@ -1130,7 +1132,7 @@ class externallib_test extends externallib_advanced_testcase {
         $record2->userid = $user1->id;
         $record2->forum = $forum1->id;
         $discussion2 = self::getDataGenerator()->get_plugin_generator('mod_forum')->create_discussion($record2);
-        sleep(1);
+        $clock->bump();
 
         // Add one reply to the discussion 2.
         $record2 = new \stdClass();
@@ -1138,7 +1140,7 @@ class externallib_test extends externallib_advanced_testcase {
         $record2->parent = $discussion2->firstpost;
         $record2->userid = $user2->id;
         $discussion2reply1 = self::getDataGenerator()->get_plugin_generator('mod_forum')->create_post($record2);
-        sleep(1);
+        $clock->bump();
 
         // Create discussion 3.
         $record3 = new \stdClass();
@@ -1146,7 +1148,7 @@ class externallib_test extends externallib_advanced_testcase {
         $record3->userid = $user1->id;
         $record3->forum = $forum1->id;
         $discussion3 = self::getDataGenerator()->get_plugin_generator('mod_forum')->create_discussion($record3);
-        sleep(1);
+        $clock->bump();
 
         // Add two replies to the discussion 3.
         $record3 = new \stdClass();
@@ -1154,7 +1156,7 @@ class externallib_test extends externallib_advanced_testcase {
         $record3->parent = $discussion3->firstpost;
         $record3->userid = $user2->id;
         $discussion3reply1 = self::getDataGenerator()->get_plugin_generator('mod_forum')->create_post($record3);
-        sleep(1);
+        $clock->bump();
 
         $record3->parent = $discussion3reply1->id;
         $record3->userid = $user3->id;
diff --git a/mod/forum/tests/generator/lib.php b/mod/forum/tests/generator/lib.php
index 2a078c1b9a4..de276702cc5 100644
--- a/mod/forum/tests/generator/lib.php
+++ b/mod/forum/tests/generator/lib.php
@@ -14,17 +14,6 @@
 // You should have received a copy of the GNU General Public License
 // along with Moodle.  If not, see <http://www.gnu.org/licenses/>.
 
-/**
- * mod_forum data generator
- *
- * @package    mod_forum
- * @category   test
- * @copyright  2012 Petr Skoda {@link http://skodak.org}
- * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
- */
-
-defined('MOODLE_INTERNAL') || die();
-
 
 /**
  * Forum module data generator class
@@ -35,7 +24,6 @@ defined('MOODLE_INTERNAL') || die();
  * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
  */
 class mod_forum_generator extends testing_module_generator {
-
     /**
      * @var int keep track of how many forum discussions have been created.
      */
@@ -51,6 +39,15 @@ class mod_forum_generator extends testing_module_generator {
      */
     protected $forumsubscriptionscount = 0;
 
+    /**
+     * Get the clock implementation to use when generating data.
+     *
+     * @return \core\clock
+     */
+    protected function get_clock(): \core\clock {
+        return \core\di::get(\core\clock::class);
+    }
+
     /**
      * To be called from data reset code only,
      * do not use in tests.
@@ -188,8 +185,14 @@ class mod_forum_generator extends testing_module_generator {
             $record['mailnow'] = "0";
         }
 
+        if (!isset($record['timecreated'])) {
+            $record['timecreated'] = $this->get_clock()->now()->getTimestamp();
+        }
+
         if (isset($record['timemodified'])) {
             $timemodified = $record['timemodified'];
+        } else {
+            $timemodified = $record['timecreated'];
         }
 
         if (!isset($record['pinned'])) {
@@ -276,11 +279,19 @@ class mod_forum_generator extends testing_module_generator {
         }
 
         if (!isset($record['created'])) {
-            $record['created'] = $time;
+            // If we are using the system clock, then revert to the time + count approach.
+            // Unfortunately a lot of Forum code relies on things not happening at the same time.
+            // See MDL-80838 for more information on this issue.
+
+            if ($this->get_clock() instanceof \core\system_clock) {
+                $record['created'] = $time;
+            } else {
+                $record['created'] = $this->get_clock()->now()->getTimestamp();
+            }
         }
 
         if (!isset($record['modified'])) {
-            $record['modified'] = $time;
+            $record['modified'] = $record['created'];
         }
 
         if (!isset($record['mailed'])) {
diff --git a/mod/forum/tests/generator_test.php b/mod/forum/tests/generator_test.php
index ba190831095..daf9c01009d 100644
--- a/mod/forum/tests/generator_test.php
+++ b/mod/forum/tests/generator_test.php
@@ -20,12 +20,12 @@ namespace mod_forum;
  * PHPUnit data generator testcase
  *
  * @package    mod_forum
- * @category   phpunit
+ * @category   test
  * @copyright  2012 Petr Skoda {@link http://skodak.org}
  * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ * @covers \mod_forum_generator
  */
-class generator_test extends \advanced_testcase {
-
+final class generator_test extends \advanced_testcase {
     public function setUp(): void {
         // We must clear the subscription caches. This has to be done both before each test, and after in case of other
         // tests using these functions.
@@ -38,7 +38,7 @@ class generator_test extends \advanced_testcase {
         \mod_forum\subscriptions::reset_forum_cache();
     }
 
-    public function test_generator() {
+    public function test_generator(): void {
         global $DB;
 
         $this->resetAfterTest(true);
@@ -52,9 +52,9 @@ class generator_test extends \advanced_testcase {
         $this->assertInstanceOf('mod_forum_generator', $generator);
         $this->assertEquals('forum', $generator->get_modulename());
 
-        $generator->create_instance(array('course'=>$course->id));
-        $generator->create_instance(array('course'=>$course->id));
-        $forum = $generator->create_instance(array('course'=>$course->id));
+        $generator->create_instance(['course' => $course->id]);
+        $generator->create_instance(['course' => $course->id]);
+        $forum = $generator->create_instance(['course' => $course->id]);
         $this->assertEquals(3, $DB->count_records('forum'));
 
         $cm = get_coursemodule_from_instance('forum', $forum->id);
@@ -65,9 +65,12 @@ class generator_test extends \advanced_testcase {
         $context = \context_module::instance($cm->id);
         $this->assertEquals($forum->cmid, $context->instanceid);
 
-        // test gradebook integration using low level DB access - DO NOT USE IN PLUGIN CODE!
-        $forum = $generator->create_instance(array('course'=>$course->id, 'assessed'=>1, 'scale'=>100));
-        $gitem = $DB->get_record('grade_items', array('courseid'=>$course->id, 'itemtype'=>'mod', 'itemmodule'=>'forum', 'iteminstance'=>$forum->id));
+        // Test gradebook integration using low level DB access - DO NOT USE IN PLUGIN CODE.
+        $forum = $generator->create_instance(['course' => $course->id, 'assessed' => 1, 'scale' => 100]);
+        $gitem = $DB->get_record(
+            'grade_items',
+            ['courseid' => $course->id, 'itemtype' => 'mod', 'itemmodule' => 'forum', 'iteminstance' => $forum->id]
+        );
         $this->assertNotEmpty($gitem);
         $this->assertEquals(100, $gitem->grademax);
         $this->assertEquals(0, $gitem->grademin);
@@ -77,7 +80,7 @@ class generator_test extends \advanced_testcase {
     /**
      * Test create_discussion.
      */
-    public function test_create_discussion() {
+    public function test_create_discussion(): void {
         global $DB;
 
         $this->resetAfterTest(true);
@@ -94,7 +97,7 @@ class generator_test extends \advanced_testcase {
         $forum = self::getDataGenerator()->create_module('forum', $record);
 
         // Add a few discussions.
-        $record = array();
+        $record = [];
         $record['course'] = $course->id;
         $record['forum'] = $forum->id;
         $record['userid'] = $user->id;
@@ -105,24 +108,29 @@ class generator_test extends \advanced_testcase {
         self::getDataGenerator()->get_plugin_generator('mod_forum')->create_discussion($record);
 
         // Check the discussions were correctly created.
-        $this->assertEquals(3, $DB->count_records_select('forum_discussions', 'forum = :forum',
-            array('forum' => $forum->id)));
+        $this->assertEquals(3, $DB->count_records_select(
+            'forum_discussions',
+            'forum = :forum',
+            ['forum' => $forum->id]
+        ));
 
-        $record['tags'] = array('Cats', 'mice');
+        $record['tags'] = ['Cats', 'mice'];
         $record = self::getDataGenerator()->get_plugin_generator('mod_forum')->create_discussion($record);
-        $this->assertEquals(array('Cats', 'mice'),
-            array_values(\core_tag_tag::get_item_tags_array('mod_forum', 'forum_posts', $record->firstpost)));
+        $this->assertEquals(
+            ['Cats', 'mice'],
+            array_values(\core_tag_tag::get_item_tags_array('mod_forum', 'forum_posts', $record->firstpost))
+        );
     }
 
     /**
      * Test create_post.
      */
-    public function test_create_post() {
+    public function test_create_post(): void {
         global $DB;
 
         $this->resetAfterTest(true);
 
-        // Create a bunch of users
+        // Create a bunch of users.
         $user1 = self::getDataGenerator()->create_user();
         $user2 = self::getDataGenerator()->create_user();
         $user3 = self::getDataGenerator()->create_user();
@@ -153,21 +161,26 @@ class generator_test extends \advanced_testcase {
 
         // Check the posts were correctly created, remember, when creating a discussion a post
         // is generated as well, so we should have 4 posts, not 3.
-        $this->assertEquals(4, $DB->count_records_select('forum_posts', 'discussion = :discussion',
-            array('discussion' => $discussion->id)));
+        $this->assertEquals(4, $DB->count_records_select(
+            'forum_posts',
+            'discussion = :discussion',
+            ['discussion' => $discussion->id]
+        ));
 
-        $record->tags = array('Cats', 'mice');
+        $record->tags = ['Cats', 'mice'];
         $record = self::getDataGenerator()->get_plugin_generator('mod_forum')->create_post($record);
-        $this->assertEquals(array('Cats', 'mice'),
-            array_values(\core_tag_tag::get_item_tags_array('mod_forum', 'forum_posts', $record->id)));
+        $this->assertEquals(
+            ['Cats', 'mice'],
+            array_values(\core_tag_tag::get_item_tags_array('mod_forum', 'forum_posts', $record->id))
+        );
     }
 
-    public function test_create_content() {
+    public function test_create_content(): void {
         global $DB;
 
         $this->resetAfterTest(true);
 
-        // Create a bunch of users
+        // Create a bunch of users.
         $user1 = self::getDataGenerator()->create_user();
         $user2 = self::getDataGenerator()->create_user();
         $user3 = self::getDataGenerator()->create_user();
@@ -177,22 +190,22 @@ class generator_test extends \advanced_testcase {
 
         // Create course and forum.
         $course = self::getDataGenerator()->create_course();
-        $forum = self::getDataGenerator()->create_module('forum', array('course' => $course));
+        $forum = self::getDataGenerator()->create_module('forum', ['course' => $course]);
 
         $generator = self::getDataGenerator()->get_plugin_generator('mod_forum');
         // This should create discussion.
         $post1 = $generator->create_content($forum);
         // This should create posts in the discussion.
-        $post2 = $generator->create_content($forum, array('parent' => $post1->id));
-        $post3 = $generator->create_content($forum, array('discussion' => $post1->discussion));
+        $post2 = $generator->create_content($forum, ['parent' => $post1->id]);
+        $post3 = $generator->create_content($forum, ['discussion' => $post1->discussion]);
         // This should create posts answering another post.
-        $post4 = $generator->create_content($forum, array('parent' => $post2->id));
+        $post4 = $generator->create_content($forum, ['parent' => $post2->id]);
         // This should create post with tags.
-        $post5 = $generator->create_content($forum, array('parent' => $post2->id, 'tags' => array('Cats', 'mice')));
+        $post5 = $generator->create_content($forum, ['parent' => $post2->id, 'tags' => ['Cats', 'mice']]);
 
-        $discussionrecords = $DB->get_records('forum_discussions', array('forum' => $forum->id));
+        $discussionrecords = $DB->get_records('forum_discussions', ['forum' => $forum->id]);
         $postrecords = $DB->get_records('forum_posts');
-        $postrecords2 = $DB->get_records('forum_posts', array('discussion' => $post1->discussion));
+        $postrecords2 = $DB->get_records('forum_posts', ['discussion' => $post1->discussion]);
         $this->assertEquals(1, count($discussionrecords));
         $this->assertEquals(5, count($postrecords));
         $this->assertEquals(5, count($postrecords2));
@@ -201,7 +214,72 @@ class generator_test extends \advanced_testcase {
         $this->assertEquals($post1->id, $postrecords[$post3->id]->parent);
         $this->assertEquals($post2->id, $postrecords[$post4->id]->parent);
 
-        $this->assertEquals(array('Cats', 'mice'),
-            array_values(\core_tag_tag::get_item_tags_array('mod_forum', 'forum_posts', $post5->id)));
+        $this->assertEquals(
+            ['Cats', 'mice'],
+            array_values(\core_tag_tag::get_item_tags_array('mod_forum', 'forum_posts', $post5->id))
+        );
+    }
+
+    public function test_create_post_time_system(): void {
+        $this->resetAfterTest(true);
+
+        $user = self::getDataGenerator()->create_user();
+        $course = $this->getDataGenerator()->create_course();
+        $forum = self::getDataGenerator()->create_module('forum', (object) [
+            'course' => $course->id,
+        ]);
+
+        $starttime = time();
+
+        // Add a discussion.
+        $discussion = self::getDataGenerator()->get_plugin_generator('mod_forum')->create_discussion((object) [
+            'course' => $course->id,
+            'forum' => $forum->id,
+            'userid' => $user->id,
+        ]);
+
+        // Add a post.
+        $post = self::getDataGenerator()->get_plugin_generator('mod_forum')->create_post((object) [
+            'discussion' => $discussion->id,
+            'userid' => $user->id,
+        ]);
+
+        $this->assertGreaterThanOrEqual($starttime, $discussion->timemodified);
+        $this->assertGreaterThanOrEqual($starttime, $post->created);
+
+        // The fallback behavior is to add the number of created posts to the current time to avoid duplicates.
+        $this->assertLessThanOrEqual(time() + 1, $discussion->timemodified);
+        $this->assertLessThanOrEqual(time() + 2, $post->created);
+    }
+
+    public function test_create_post_time_frozen(): void {
+        $this->resetAfterTest(true);
+
+        $clock = $this->mock_clock_with_frozen(100);
+
+        $user = self::getDataGenerator()->create_user();
+        $course = $this->getDataGenerator()->create_course();
+        $forum = self::getDataGenerator()->create_module('forum', (object) [
+            'course' => $course->id,
+        ]);
+
+        $starttime = time();
+
+        // Add a discussion.
+        $discussion = self::getDataGenerator()->get_plugin_generator('mod_forum')->create_discussion((object) [
+            'course' => $course->id,
+            'forum' => $forum->id,
+            'userid' => $user->id,
+        ]);
+        $this->assertEquals(100, $discussion->timemodified);
+
+        // Add a post.
+        $clock->set_to(200);
+        $post = self::getDataGenerator()->get_plugin_generator('mod_forum')->create_post((object) [
+            'discussion' => $discussion->id,
+            'userid' => $user->id,
+        ]);
+
+        $this->assertEquals(200, $post->created);
     }
 }