warning on notifications of student dashboard

When I click on notifications link on student dashboard, a php warning appears:
Warning: A non-numeric value encountered in ***\wp-content\plugins\lifterlms\includes\abstracts\llms.abstract.notification.view.php on line 292:

287 public function get_date_display( $max_days = 5 ) {
288
289 $now = current_time( ‘timestamp’ );
290 $created = $this->get_date( ‘created’, ‘U’ );
291
292 if ( ( $now – $created ) <= ( $max_days * DAY_IN_SECONDS ) ) {
293
294 return sprintf( _x( ‘About %s ago’, ‘relative date display’,
294 ‘lifterlms’ ), $this->get_date_relative( ‘created’ ) );
295
296 }
297
298 return $this->get_date( ‘created’ );

What does it mean and how can I fix it?

Thanks!