diff --git a/src/wp-includes/rest-api/endpoints/class-wp-rest-templates-controller.php b/src/wp-includes/rest-api/endpoints/class-wp-rest-templates-controller.php
index b821ca09453e3..cb901c5637f0e 100644
--- a/src/wp-includes/rest-api/endpoints/class-wp-rest-templates-controller.php
+++ b/src/wp-includes/rest-api/endpoints/class-wp-rest-templates-controller.php
@@ -668,6 +668,8 @@ protected function prepare_item_for_database( $request ) {
 	 * @since 5.9.0 Renamed `$template` to `$item` to match parent class for PHP 8 named parameter support.
 	 * @since 6.3.0 Added `modified` property to the response.
 	 * @since 7.1.0 Added `date` property to the response.
+	 * @since 7.2.0 The `modified` property is `null` for templates that have no
+	 *              modification date.
 	 *
 	 * @param WP_Block_Template $item    Template instance.
 	 * @param WP_REST_Request   $request Request object.
@@ -776,7 +778,12 @@ public function prepare_item_for_response( $item, $request ) {
 		}
 
 		if ( rest_is_field_included( 'modified', $fields ) ) {
-			$data['modified'] = mysql_to_rfc3339( $template->modified );
+			/*
+			 * File-backed templates have no modification date. Return `null` in that
+			 * case, as `mysql_to_rfc3339()` would return `false` for an empty value,
+			 * which the schema does not allow.
+			 */
+			$data['modified'] = empty( $template->modified ) ? null : mysql_to_rfc3339( $template->modified );
 		}
 
 		if ( rest_is_field_included( 'date', $fields ) ) {
@@ -1154,7 +1161,7 @@ public function get_item_schema() {
 				),
 				'modified'        => array(
 					'description' => __( "The date the template was last modified, in the site's timezone." ),
-					'type'        => 'string',
+					'type'        => array( 'string', 'null' ),
 					'format'      => 'date-time',
 					'context'     => array( 'view', 'edit' ),
 					'readonly'    => true,
diff --git a/tests/phpunit/tests/rest-api/wpRestTemplatesController.php b/tests/phpunit/tests/rest-api/wpRestTemplatesController.php
index 42eed8dfa9c35..2f4f6e82fc1e5 100644
--- a/tests/phpunit/tests/rest-api/wpRestTemplatesController.php
+++ b/tests/phpunit/tests/rest-api/wpRestTemplatesController.php
@@ -638,6 +638,39 @@ public function test_get_item_from_registry() {
 		$this->assertSame( 404, $response->get_status(), 'Fetching an unregistered template should return 404.' );
 	}
 
+	/**
+	 * A file-backed template has no modification date, which should be exposed as
+	 * `null` rather than the `false` returned by `mysql_to_rfc3339()`.
+	 *
+	 * @ticket 65730
+	 * @covers WP_REST_Templates_Controller::prepare_item_for_response
+	 */
+	public function test_get_item_modified_is_null_for_file_backed_template() {
+		wp_set_current_user( self::$admin_id );
+		switch_theme( 'block-theme' );
+
+		$request  = new WP_REST_Request( 'GET', '/wp/v2/templates/block-theme//page-home' );
+		$response = rest_get_server()->dispatch( $request );
+		$data     = $response->get_data();
+
+		$this->assertSame( 200, $response->get_status(), 'Fetching a file-backed template should return 200.' );
+		$this->assertNull( $data['modified'], 'The modified date should be null for a file-backed template.' );
+	}
+
+	/**
+	 * @ticket 65730
+	 * @covers WP_REST_Templates_Controller::get_item_schema
+	 */
+	public function test_modified_schema_allows_null() {
+		$request    = new WP_REST_Request( 'OPTIONS', '/wp/v2/templates' );
+		$response   = rest_get_server()->dispatch( $request );
+		$data       = $response->get_data();
+		$properties = $data['schema']['properties'];
+
+		$this->assertArrayHasKey( 'modified', $properties );
+		$this->assertSame( array( 'string', 'null' ), $properties['modified']['type'] );
+	}
+
 	/**
 	 * @ticket 54507
 	 * @dataProvider data_sanitize_template_id