use IO::Socket;

print GetNTPTime('ntp1.cs.wisc.edu');

sub GetNTPTime{
	my $host = shift;
	my $timeout = 2;
	my $sock = IO::Socket::INET->new(
		Proto => 'udp',
		PeerPort => 123,
		PeerAddr => $host,
	  Timeout => $timeout
	)	or do {warn "Can't contact $host\n"; return 0};
	my $ntp_msg = pack("B8 C3 N11", '00100011', (0)x14);
	$sock->send($ntp_msg) or do {warn "Can't send NTP msg."; return 0};
	my $rin='';
	vec($rin, fileno($sock), 1) = 1;
	select(my $rout=$rin, undef, my $eout=$rin, $timeout) or do {warn "No answer from $host"; return 0};
	$sock->recv($ntp_msg, length($ntp_msg))
		or do {warn "Receive error from $host: ($!)"; return 0};
	my ($LIVNMode, $Stratum, $Poll, $Precision,
			$RootDelay, $RootDispersion, $RefIdentifier,
			$Reference, $ReferenceF, $Original, $OriginalF,
			$Receive, $ReceiveF, $Transmit, $TransmitF
		)	= unpack('a C2 c1 N8 N N B32', $ntp_msg);
	$Receive -= 2208988800;
	my $NTPTime = $Receive + $ReceiveF / 65536 / 65536;
	$RefIdentifier = unpack('A4', $RefIdentifier);
	my $LI = vec($LIVNMode, 3, 2);
	my $VN = unpack("C", $LIVNMode & "\x38") >> 3;
	my $Mode = unpack("C", $LIVNMode & "\x07");
	$sock->close;
	if ($RefIdentifier ne '1280') {
		return $NTPTime
	} else {
		return 0
	}
}