I want to print a message in post ” After one month from publish date”

Custom conditional I wrote for my theme (mostly to kick some widgets and ads from newer posts), works nicely:

function is_old_post( $days = 14 ) {

    if( !is_single() )
       return false;

    $id = get_the_ID();
    $date = get_post_field( 'post_date', $id );

    if( strtotime( $date ) < strtotime( "-{$days} days" ) )
        return true;

   return false;
}