sub sec2string {
  my $sec = shift;
  my @str;
  
  for $d (60, 60, 24) {
     my $m = $sec % $d; $sec /= $d;
     $m = $m < 10 ? '0'.$m;
     unshift @str,$m;
  }
  return "$sec:" . join ":",@str;
}