public String seconds(Duration duration) {
    double seconds = duration.toNanos() * 1d / Duration.ofSeconds(1).toNanos();
    return decimal(seconds < 1 ? seconds : Math.round(seconds)) + "s";
  }
Returns a duration formatted as fractional seconds with 1 decimal point.
planetiler-core/src/main/java/com/onthegomap/planetiler/util/Format.java