diff --git a/.github/changelog/fix-c2s-activity-date-fallback b/.github/changelog/fix-c2s-activity-date-fallback
new file mode 100644
index 0000000000..d4e38cb445
--- /dev/null
+++ b/.github/changelog/fix-c2s-activity-date-fallback
@@ -0,0 +1,4 @@
+Significance: patch
+Type: fixed
+
+Activities read from the inbox and outbox C2S endpoints now use the local record date as a fallback when no publish date is set, so client apps show consistent timestamps.
diff --git a/includes/collection/class-inbox.php b/includes/collection/class-inbox.php
index 55778a6b52..92d12d6ea8 100644
--- a/includes/collection/class-inbox.php
+++ b/includes/collection/class-inbox.php
@@ -225,6 +225,60 @@ public static function get_by_guid( $guid ) {
 		return \get_post( $post_id );
 	}
 
+	/**
+	 * Reconstruct the Activity stored in an inbox item.
+	 *
+	 * Hydrates the JSON from `post_content` into an Activity object and, when
+	 * the activity is missing `published`/`updated`, falls back to the inbox
+	 * row's `post_date_gmt`/`post_modified_gmt`. The CPT timestamps record when
+	 * we received the activity, which is the natural fallback when the remote
+	 * sender omitted those fields.
+	 *
+	 * @param int|\WP_Post $inbox_item The inbox post or post ID.
+	 *
+	 * @return Activity|\WP_Error The Activity object or WP_Error.
+	 */
+	public static function get_activity( $inbox_item ) {
+		$inbox_item = \get_post( $inbox_item );
+
+		if ( ! $inbox_item || self::POST_TYPE !== $inbox_item->post_type ) {
+			return new \WP_Error(
+				'activitypub_inbox_item_not_found',
+				\__( 'Inbox item not found.', 'activitypub' ),
+				array( 'status' => 404 )
+			);
+		}
+
+		$data = \json_decode( $inbox_item->post_content, true );
+
+		if ( ! \is_array( $data ) ) {
+			return new \WP_Error(
+				'activitypub_inbox_item_invalid',
+				\__( 'Inbox item is not a valid activity.', 'activitypub' ),
+				array( 'status' => 500 )
+			);
+		}
+
+		$activity = Activity::init_from_array( $data );
+
+		if ( \is_wp_error( $activity ) ) {
+			return $activity;
+		}
+
+		$post_date_gmt     = empty( $inbox_item->post_date_gmt ) || '0000-00-00 00:00:00' === $inbox_item->post_date_gmt ? '' : $inbox_item->post_date_gmt;
+		$post_modified_gmt = empty( $inbox_item->post_modified_gmt ) || '0000-00-00 00:00:00' === $inbox_item->post_modified_gmt ? '' : $inbox_item->post_modified_gmt;
+
+		if ( ! $activity->get_published() && $post_date_gmt ) {
+			$activity->set_published( \gmdate( ACTIVITYPUB_DATE_TIME_RFC3339, \strtotime( $post_date_gmt ) ) );
+		}
+
+		if ( ! $activity->get_updated() && $post_modified_gmt && $post_modified_gmt > $post_date_gmt ) {
+			$activity->set_updated( \gmdate( ACTIVITYPUB_DATE_TIME_RFC3339, \strtotime( $post_modified_gmt ) ) );
+		}
+
+		return $activity;
+	}
+
 	/**
 	 * Undo a received activity.
 	 *
diff --git a/includes/collection/class-outbox.php b/includes/collection/class-outbox.php
index f30c5ff3d2..463ef35b95 100644
--- a/includes/collection/class-outbox.php
+++ b/includes/collection/class-outbox.php
@@ -395,8 +395,45 @@ public static function get_activity( $outbox_item ) {
 			$activity->set_object( $activity_object );
 		}
 
-		if ( 'Update' === $type ) {
-			$activity->set_updated( gmdate( ACTIVITYPUB_DATE_TIME_RFC3339, strtotime( $outbox_item->post_modified ) ) );
+		/*
+		 * Fall back to the outbox row's timestamps when the hydrated activity is
+		 * missing `published`/`updated`. The CPT row is the authoritative record
+		 * of when the activity was emitted, so dropping it on the floor here
+		 * means downstream consumers (federation, REST listings, audit tooling)
+		 * see a date-less activity even though we know exactly when it left.
+		 *
+		 * `Outbox::add` inserts with `post_status = 'pending'`, which leaves the
+		 * `_gmt` columns as the `0000-00-00 00:00:00` sentinel while the local
+		 * columns are populated via `current_time( 'mysql' )`. The Dispatcher
+		 * reads the activity while the row is still pending, so the GMT columns
+		 * are derived from the local columns when the sentinel is present —
+		 * synthesizing `1970-01-01T00:00:00Z` would be worse than the field
+		 * being empty, and `Update` activities would lose `updated` on
+		 * federation if we relied on the GMT column alone.
+		 */
+		$post_date_gmt = $outbox_item->post_date_gmt;
+		if ( empty( $post_date_gmt ) || '0000-00-00 00:00:00' === $post_date_gmt ) {
+			$post_date_gmt = empty( $outbox_item->post_date ) || '0000-00-00 00:00:00' === $outbox_item->post_date
+				? ''
+				: \get_gmt_from_date( $outbox_item->post_date );
+		}
+
+		$post_modified_gmt = $outbox_item->post_modified_gmt;
+		if ( empty( $post_modified_gmt ) || '0000-00-00 00:00:00' === $post_modified_gmt ) {
+			$post_modified_gmt = empty( $outbox_item->post_modified ) || '0000-00-00 00:00:00' === $outbox_item->post_modified
+				? ''
+				: \get_gmt_from_date( $outbox_item->post_modified );
+		}
+
+		if ( ! $activity->get_published() && $post_date_gmt ) {
+			$activity->set_published( \gmdate( ACTIVITYPUB_DATE_TIME_RFC3339, \strtotime( $post_date_gmt ) ) );
+		}
+
+		if ( ! $activity->get_updated() && $post_modified_gmt ) {
+			$needs_updated = ( 'Update' === $type ) || ( $post_modified_gmt > $post_date_gmt );
+			if ( $needs_updated ) {
+				$activity->set_updated( \gmdate( ACTIVITYPUB_DATE_TIME_RFC3339, \strtotime( $post_modified_gmt ) ) );
+			}
 		}
 
 		/**
diff --git a/includes/rest/class-actors-inbox-controller.php b/includes/rest/class-actors-inbox-controller.php
index 855e9e009e..d46120e2ae 100644
--- a/includes/rest/class-actors-inbox-controller.php
+++ b/includes/rest/class-actors-inbox-controller.php
@@ -207,7 +207,13 @@ public function get_items( $request ) {
 				continue;
 			}
 
-			$response['orderedItems'][] = $this->prepare_item_for_response( $inbox_item, $request );
+			$item = $this->prepare_item_for_response( $inbox_item, $request );
+
+			if ( \is_wp_error( $item ) ) {
+				continue;
+			}
+
+			$response['orderedItems'][] = $item;
 		}
 
 		$response = $this->prepare_collection_response( $response, $request );
@@ -249,12 +255,16 @@ public function get_items( $request ) {
 	 *
 	 * @param mixed            $item    WordPress representation of the item.
 	 * @param \WP_REST_Request $request Request object.
-	 * @return array Response object on success.
+	 * @return array|\WP_Error Response object on success, or WP_Error object on failure.
 	 */
 	public function prepare_item_for_response( $item, $request ) { // phpcs:ignore VariableAnalysis.CodeAnalysis.VariableAnalysis.UnusedVariable
-		$activity = \json_decode( $item->post_content, true );
+		$activity = Inbox::get_activity( $item->ID );
+
+		if ( \is_wp_error( $activity ) ) {
+			return $activity;
+		}
 
-		return $activity;
+		return $activity->to_array( false );
 	}
 
 	/**
diff --git a/tests/phpunit/tests/includes/collection/class-test-inbox.php b/tests/phpunit/tests/includes/collection/class-test-inbox.php
index cb64c292fc..f90269b709 100644
--- a/tests/phpunit/tests/includes/collection/class-test-inbox.php
+++ b/tests/phpunit/tests/includes/collection/class-test-inbox.php
@@ -1283,4 +1283,119 @@ public function test_purge_returns_deleted_count() {
 		// Should return exact count of deleted posts.
 		$this->assertEquals( 15, $deleted );
 	}
+
+	/**
+	 * Helper: build a Create activity and store it in the inbox.
+	 *
+	 * @return int Inbox post ID.
+	 */
+	private function add_test_activity_to_inbox() {
+		$object = new Base_Object();
+		$object->set_id( 'https://remote.example.com/objects/' . \wp_generate_uuid4() );
+		$object->set_type( 'Note' );
+		$object->set_content( 'fallback-published-test' );
+
+		$activity = new Activity();
+		$activity->set_id( 'https://remote.example.com/activities/' . \wp_generate_uuid4() );
+		$activity->set_type( 'Create' );
+		$activity->set_actor( 'https://remote.example.com/users/testuser' );
+		$activity->set_object( $object );
+
+		$id = Inbox::add( $activity, 1 );
+		$this->assertIsInt( $id );
+
+		return $id;
+	}
+
+	/**
+	 * `Inbox::get_activity` fills `published` from `post_date_gmt` when the stored JSON has none.
+	 *
+	 * @covers ::get_activity
+	 */
+	public function test_get_activity_fills_missing_published_from_post_date_gmt() {
+		$id = $this->add_test_activity_to_inbox();
+
+		$post = \get_post( $id );
+		$raw  = \json_decode( $post->post_content, true );
+		unset( $raw['published'] );
+		\wp_update_post(
+			array(
+				'ID'           => $id,
+				'post_content' => \wp_slash( \wp_json_encode( $raw ) ),
+			)
+		);
+
+		$activity = Inbox::get_activity( $id );
+		$this->assertInstanceOf( Activity::class, $activity );
+
+		$post     = \get_post( $id );
+		$expected = \gmdate( 'Y-m-d\TH:i:s\Z', \strtotime( $post->post_date_gmt ) );
+		$this->assertEquals( $expected, $activity->get_published() );
+	}
+
+	/**
+	 * `Inbox::get_activity` preserves a `published` value that was present in the stored JSON.
+	 *
+	 * @covers ::get_activity
+	 */
+	public function test_get_activity_preserves_existing_published() {
+		$id = $this->add_test_activity_to_inbox();
+
+		$frozen           = '2019-06-07T08:09:10Z';
+		$post             = \get_post( $id );
+		$raw              = \json_decode( $post->post_content, true );
+		$raw['published'] = $frozen;
+		\wp_update_post(
+			array(
+				'ID'           => $id,
+				'post_content' => \wp_slash( \wp_json_encode( $raw ) ),
+			)
+		);
+
+		$activity = Inbox::get_activity( $id );
+		$this->assertEquals( $frozen, $activity->get_published() );
+	}
+
+	/**
+	 * Invalid post ID returns WP_Error.
+	 *
+	 * @covers ::get_activity
+	 */
+	public function test_get_activity_invalid_post_id_returns_error() {
+		$result = Inbox::get_activity( 999999999 );
+		$this->assertInstanceOf( \WP_Error::class, $result );
+		$this->assertEquals( 'activitypub_inbox_item_not_found', $result->get_error_code() );
+	}
+
+	/**
+	 * A non-inbox post (wrong post_type) returns WP_Error.
+	 *
+	 * @covers ::get_activity
+	 */
+	public function test_get_activity_wrong_post_type_returns_error() {
+		$regular_post_id = self::factory()->post->create();
+		$result          = Inbox::get_activity( $regular_post_id );
+		$this->assertInstanceOf( \WP_Error::class, $result );
+		$this->assertEquals( 'activitypub_inbox_item_not_found', $result->get_error_code() );
+	}
+
+	/**
+	 * Corrupt post_content returns WP_Error.
+	 *
+	 * @covers ::get_activity
+	 */
+	public function test_get_activity_corrupt_json_returns_error() {
+		$id = $this->add_test_activity_to_inbox();
+
+		\wp_update_post(
+			array(
+				'ID'           => $id,
+				'post_content' => '{not valid json',
+			)
+		);
+
+		$result = Inbox::get_activity( $id );
+		$this->assertInstanceOf( \WP_Error::class, $result );
+		$this->assertEquals( 'activitypub_inbox_item_invalid', $result->get_error_code() );
+	}
 }
diff --git a/tests/phpunit/tests/includes/collection/class-test-outbox.php b/tests/phpunit/tests/includes/collection/class-test-outbox.php
index 80eaa5fad4..215acedb54 100644
--- a/tests/phpunit/tests/includes/collection/class-test-outbox.php
+++ b/tests/phpunit/tests/includes/collection/class-test-outbox.php
@@ -653,16 +653,259 @@ public function test_update_activity_has_updated_attribute() {
 		$id = \Activitypub\add_to_outbox( $object, 'Update', 1 );
 		$this->assertNotFalse( $id );
 
+		global $wpdb;
+		$now = \gmdate( 'Y-m-d H:i:s' );
+		// phpcs:ignore WordPress.DB.DirectDatabaseQuery
+		$wpdb->update(
+			$wpdb->posts,
+			array(
+				'post_date'         => $now,
+				'post_date_gmt'     => $now,
+				'post_modified'     => $now,
+				'post_modified_gmt' => $now,
+			),
+			array( 'ID' => $id )
+		);
+		\clean_post_cache( $id );
+
 		// Get the activity from the outbox.
 		$activity = Outbox::get_activity( $id );
 		$this->assertNotInstanceOf( \WP_Error::class, $activity );
 
 		// Verify the updated attribute is set and matches the post's modified date.
 		$post             = \get_post( $id );
-		$expected_updated = \gmdate( 'Y-m-d\TH:i:s\Z', \strtotime( $post->post_modified ) );
+		$expected_updated = \gmdate( 'Y-m-d\TH:i:s\Z', \strtotime( $post->post_modified_gmt ) );
 		$this->assertEquals( $expected_updated, $activity->get_updated() );
 	}
 
+	/**
+	 * Stored Create activity without `published` gets it from the outbox row.
+	 *
+	 * @covers ::get_activity
+	 */
+	public function test_get_activity_fills_missing_published_from_post_date_gmt() {
+		$object = $this->get_dummy_activity_object();
+		$id     = \Activitypub\add_to_outbox( $object, 'Create', 1 );
+		$this->assertNotFalse( $id );
+
+		// Strip `published` from the stored JSON to simulate a transformer that omitted it.
+		$post = \get_post( $id );
+		$raw  = \json_decode( $post->post_content, true );
+		unset( $raw['published'] );
+		\wp_update_post(
+			array(
+				'ID'           => $id,
+				'post_content' => \wp_slash( \wp_json_encode( $raw ) ),
+			)
+		);
+
+		// Populate post_date_gmt explicitly — Outbox::add inserts with status=pending,
+		// so WordPress leaves the GMT date as 0000-00-00 00:00:00 and the fallback
+		// would otherwise round-trip an epoch-zero value.
+		global $wpdb;
+		$now = \gmdate( 'Y-m-d H:i:s' );
+		// phpcs:ignore WordPress.DB.DirectDatabaseQuery
+		$wpdb->update(
+			$wpdb->posts,
+			array(
+				'post_date'     => $now,
+				'post_date_gmt' => $now,
+			),
+			array( 'ID' => $id )
+		);
+		\clean_post_cache( $id );
+
+		$activity = Outbox::get_activity( $id );
+		$this->assertNotInstanceOf( \WP_Error::class, $activity );
+
+		$post     = \get_post( $id );
+		$expected = \gmdate( 'Y-m-d\TH:i:s\Z', \strtotime( $post->post_date_gmt ) );
+		$this->assertEquals( $expected, $activity->get_published() );
+	}
+
+	/**
+	 * Stored activity that already has a `published` value is not overwritten.
+	 *
+	 * @covers ::get_activity
+	 */
+	public function test_get_activity_preserves_existing_published() {
+		$object = $this->get_dummy_activity_object();
+		$id     = \Activitypub\add_to_outbox( $object, 'Create', 1 );
+		$this->assertNotFalse( $id );
+
+		$frozen           = '2020-01-02T03:04:05Z';
+		$post             = \get_post( $id );
+		$raw              = \json_decode( $post->post_content, true );
+		$raw['published'] = $frozen;
+		\wp_update_post(
+			array(
+				'ID'           => $id,
+				'post_content' => \wp_slash( \wp_json_encode( $raw ) ),
+			)
+		);
+
+		$activity = Outbox::get_activity( $id );
+		$this->assertEquals( $frozen, $activity->get_published() );
+	}
+
+	/**
+	 * Zero-sentinel `post_date_gmt` must not synthesize a 1970-01-01 published date.
+	 *
+	 * @covers ::get_activity
+	 */
+	public function test_get_activity_does_not_synthesize_epoch_published_from_zero_date() {
+		$object = $this->get_dummy_activity_object();
+		$id     = \Activitypub\add_to_outbox( $object, 'Create', 1 );
+		$this->assertNotFalse( $id );
+
+		// Strip `published` from the stored JSON.
+		$post = \get_post( $id );
+		$raw  = \json_decode( $post->post_content, true );
+		unset( $raw['published'] );
+		\wp_update_post(
+			array(
+				'ID'           => $id,
+				'post_content' => \wp_slash( \wp_json_encode( $raw ) ),
+			)
+		);
+
+		// Force the zero sentinel that pending outbox rows actually have.
+		global $wpdb;
+		// phpcs:ignore WordPress.DB.DirectDatabaseQuery
+		$wpdb->update(
+			$wpdb->posts,
+			array(
+				'post_date'         => '0000-00-00 00:00:00',
+				'post_date_gmt'     => '0000-00-00 00:00:00',
+				'post_modified'     => '0000-00-00 00:00:00',
+				'post_modified_gmt' => '0000-00-00 00:00:00',
+			),
+			array( 'ID' => $id )
+		);
+		\clean_post_cache( $id );
+
+		$activity = Outbox::get_activity( $id );
+		$this->assertEmpty( $activity->get_published() );
+		$this->assertEmpty( $activity->get_updated() );
+	}
+
+	/**
+	 * Pending outbox row (sentinel GMT, populated local) — the Dispatcher reads
+	 * the row in this state, so an `Update` activity must still federate with
+	 * `updated` derived from `post_modified` via `get_gmt_from_date()`.
+	 *
+	 * @covers ::get_activity
+	 */
+	public function test_get_activity_derives_gmt_from_local_for_pending_update_row() {
+		$object = $this->get_dummy_activity_object();
+		$id     = \Activitypub\add_to_outbox( $object, 'Update', 1 );
+		$this->assertNotFalse( $id );
+
+		// Reproduce the column state of a freshly-added, still-pending outbox row:
+		// local columns populated, GMT columns left as the zero sentinel.
+		$local = \current_time( 'mysql' );
+		global $wpdb;
+		// phpcs:ignore WordPress.DB.DirectDatabaseQuery
+		$wpdb->update(
+			$wpdb->posts,
+			array(
+				'post_date'         => $local,
+				'post_date_gmt'     => '0000-00-00 00:00:00',
+				'post_modified'     => $local,
+				'post_modified_gmt' => '0000-00-00 00:00:00',
+			),
+			array( 'ID' => $id )
+		);
+		\clean_post_cache( $id );
+
+		$activity = Outbox::get_activity( $id );
+		$this->assertNotInstanceOf( \WP_Error::class, $activity );
+
+		$expected = \gmdate( 'Y-m-d\TH:i:s\Z', \strtotime( \get_gmt_from_date( $local ) ) );
+		$this->assertEquals( $expected, $activity->get_updated() );
+		$this->assertEquals( $expected, $activity->get_published() );
+	}
+
+	/**
+	 * Non-Update activity whose outbox row was later modified picks up `updated` from `post_modified_gmt`.
+	 *
+	 * @covers ::get_activity
+	 */
+	public function test_get_activity_fills_updated_when_row_modified() {
+		$object = $this->get_dummy_activity_object();
+		$id     = \Activitypub\add_to_outbox( $object, 'Create', 1 );
+		$this->assertNotFalse( $id );
+
+		global $wpdb;
+		$earlier = \gmdate( 'Y-m-d H:i:s', \strtotime( '-1 hour' ) );
+		// phpcs:ignore WordPress.DB.DirectDatabaseQuery
+		$wpdb->update(
+			$wpdb->posts,
+			array(
+				'post_date'     => $earlier,
+				'post_date_gmt' => $earlier,
+			),
+			array( 'ID' => $id )
+		);
+
+		// Bump post_modified_gmt to be strictly later than post_date_gmt.
+		$later = \gmdate( 'Y-m-d H:i:s', \strtotime( '+1 hour' ) );
+		// phpcs:ignore WordPress.DB.DirectDatabaseQuery
+		$wpdb->update(
+			$wpdb->posts,
+			array(
+				'post_modified'     => $later,
+				'post_modified_gmt' => $later,
+			),
+			array( 'ID' => $id )
+		);
+		\clean_post_cache( $id );
+
+		$activity = Outbox::get_activity( $id );
+		$expected = \gmdate( 'Y-m-d\TH:i:s\Z', \strtotime( $later ) );
+		$this->assertEquals( $expected, $activity->get_updated() );
+	}
+
+	/**
+	 * Non-Update activity that was never modified must not synthesize an `updated` field.
+	 *
+	 * @covers ::get_activity
+	 */
+	public function test_get_activity_leaves_updated_empty_when_not_modified() {
+		$object = $this->get_dummy_activity_object();
+		$id     = \Activitypub\add_to_outbox( $object, 'Create', 1 );
+		$this->assertNotFalse( $id );
+
+		global $wpdb;
+		$now = \gmdate( 'Y-m-d H:i:s' );
+		// phpcs:ignore WordPress.DB.DirectDatabaseQuery
+		$wpdb->update(
+			$wpdb->posts,
+			array(
+				'post_date'     => $now,
+				'post_date_gmt' => $now,
+			),
+			array( 'ID' => $id )
+		);
+		\clean_post_cache( $id );
+		$post = \get_post( $id );
+
+		// Force post_modified_gmt == post_date_gmt.
+		// phpcs:ignore WordPress.DB.DirectDatabaseQuery
+		$wpdb->update(
+			$wpdb->posts,
+			array(
+				'post_modified'     => $post->post_date,
+				'post_modified_gmt' => $post->post_date_gmt,
+			),
+			array( 'ID' => $id )
+		);
+		\clean_post_cache( $id );
+
+		$activity = Outbox::get_activity( $id );
+		$this->assertEmpty( $activity->get_updated() );
+	}
+
 	/**
 	 * Test purge method with more than 20 posts.
 	 *