Why not keep the time synchronized by running ntp on them?

This, or use some sort of state enforcement tool like Salt.

#!/usr/bin/env perl
use IO::Socket::INET;
 
# auto-flush on socket
$| = 1;
 
# creating a listening socket
my $socket = new IO::Socket::INET (
    LocalHost => '0.0.0.0',
    LocalPort => '5101',
    Proto => 'tcp',
    Listen => 10,
    Reuse => 1
);
die "cannot create socket $!\n" unless $socket;
print "server waiting for client connection on port 5101\n";
 
while(1)
{
    # waiting for a new client connection
    my $client_socket = $socket->accept();
 
    # get information about a newly connected client
    my $client_address = $client_socket->peerhost();
    my $client_port = $client_socket->peerport();
    print "connection from $client_address:$client_port\n";
 
    # read up to 16 characters from the connected client
    my $theirtime = 0;
    $client_socket->recv($theirtime, 16);
    print "received data: $theirtime\n";
    $theirtime =~s/[^\d]//g; # untaint
 
    # write response data to the connected client
    my $their_drift = time - $theirtime ;
    $client_socket->send($their_drift);
 
    # notify client that response has been sent
    shutdown($client_socket, 1);
}
 
$socket->close();

__END__

Problem

Your program has forked and you want to tell the other end that you're
+ done sending data. You've tried close on the socket, but the remote 
+end never gets an EOF or SIGPIPE.
Solution

Use shutdown:

shutdown(SOCKET, 0);                # I/we have stopped reading data
shutdown(SOCKET, 1);                # I/we have stopped writing data
shutdown(SOCKET, 2);                # I/we have stopped using this soc
+ket
[download]

can be tested with:

(perl -e 'print time';sleep 2) | telnet 127.0.0.1 5101 2>/dev/null|tai
+l -1
[download]

try time+3 to simulate time drifting...

Addendum: a small perl client:

#!/usr/bin/env perl

use IO::Socket::INET;
 
# auto-flush on socket
$| = 1;
 
# create a connecting socket
my $socket = new IO::Socket::INET (
    PeerHost => 'localhost',
    PeerPort => '5101',
    Proto => 'tcp',
);
die "cannot connect to the server $!\n" unless $socket;
print "connected to the server\n";
 
# data to send to a server
my $mytime = time;
my $size = $socket->send($mytime);
print "sent data of length $size\n";
 
# notify server that request has been sent
shutdown($socket, 1);
 
# receive a response of up to 256 characters from server
my $response = "";
$socket->recv($response, 256);
print "received response: $response\n";
 
$socket->close();

my $threshold=5; # seconds
if(abs($response)>$threshold){
    exit 1;
}else{
    exit 0;
}
[download]

edit: forgot g in the untaint...

forgot the hashbang and the "Listen => 5" should be 10 to be able to service all the machines at once. Also: use strict and warnings... Depending on the the connection time, you might want to add fallbacks, like a retry to create a socket, counting the seconds it took to get the anwser, and maybe substract half of this time from your result. Even use Time::HiRes; to get better data... But your thresholds seem to be very tolerant, so keeping it as integers is a quick duktape/chewingum solution while you roll out your ntp thing..

If you have systemd read this: systemd-timesyncd

Set up 1 server that gets it's time from the internet, and all others should get their time from that server

Ah, forgotten about this, could be useful:

search google for nagios and check_ntp_time or check_ntp for some perl code that checks the drift or the status of the NTP server.
Nagios Plugins come with check_ntp.pl (although it works, others say it is deprecated... in favour of faster C compiled ones... I still like the perl version more)

check_ntp_time.pl download

ntp monitors searchpage

Addendum, while on Debian, to synch your time,

ntpdate install and usage

Running ntp while on a laptop (and stopping it while on charge/starting it while charging)