Zusätzliche Links:

http://www.kortstock.de/dokus/eckert/zeitsynchronisation.html

http://ntp.robertkehl.de/

For closed computer networks, a relative time source will function adequately enough, however, if communication with another network or transactions over the Internet is required then it is important that the networks are synchronised to the same time otherwise the above mentioned problems can occur.

But how do you know what time another network is running? Coordinated Universal Time (UTC) is a global time scale, developed to enable computer networks all over the world to synchronise to the exact same time.

UTC arose after the development of atomic clocks. Atomic clocks use the resonance of an atom (normally caesium which resonates at 9,192,631,770 every second) to maintain time. As this resonance never changes, atomic clocks can maintain time for millions of years without losing a second.

Unfortunately, atomic clocks are highly expensive and are normally only found in large-scale physics laboratories. However, the time told by these machines is readily available to synchronise to from several sources. The easiest method is to use a dedicated NTP time server that receives a time signal from either the GPS network or a specialist national radio broadcast.

There are however, a few important aspects about installing a NTP GPS server that are often overlooked by some technicians.

The GPS signal works by line-of-sight, that is to say that the satellite has to be visible to the antenna. Obviously as GPS satellites are-in orbit that means that the antenna has to have a good view of the sky. It is also important to keep the antenna away from sources of interference such as satellite dishes or generators.

The cable length of the antenna is also important. The maximum cable length without any amplification device is 1000 metres

Whilst most NTP GPS servers can locate a satellite in a matter of minutes In order to maintain a stable satellite fix, most NTP GPS servers need to be left on for at least 48 hours to ensure stability.

Configuration of a NTP GPS server can be done over the Internet in browser window, alternatively some dedicated devices come with LCD screen and external configuration controls.

The Inter-Range Instrumentation Group (IRIG) time server is an important technology providing accurate network time synchronization. Time-critical operations - militaries, hospitals and financial services - depend on the highly precise data time stamps.

The Network Time Protocol (NTP) is a technological agreement establishing a common standard for synchronizing clocks, time and data based on the Universal Time Coordinated (UTC), which is calculated by the Greenwich, England Observatory Clock. These universal time stamp standards ensure that computer data transmission is organized in proper sequences.

Most computer clocks are inaccurate. They are set, by estimation, to within a couple of minutes of local time. The backup electrical currents are not always reliable. If the computer malfunctions, the clock could lose time. Some computer clocks only "run" when the computer is "on". The IRIG time server overcomes individual computer inaccuracy.

An IRIG time server establishes time synchronization and network accuracy using the more precise format of year, day, hour, minute, second, microsecond and millisecond coordinating with the UTC. Time synchronization gains importance for file organization as data flows in from many different sources.

While some firms may go onto the Internet for network time synchronization, this can cause a security risk by going outside the company "firewall". This procedure is also less accurate due to data transmission time delays.

Installing an IRIG time server within your firewall, can protect your network's security and ensure more accurate time stamps. The "most accurate time gives a company an advantage over its competition."

The time is reported every fifteen minutes by off air radio transmission and relayed by the GPS satellite system. Every clock in the world is set according to the atomic clock and an MSF NTP server sets the time with each transmission. The antenna of the MSF NTP server is located away from the computer itself to protect the server from radio interference.

MSF is the call sign for the original transmitter in Rugby that transmitted MSF MSF MSF every fifteen minutes in Morse code followed by the time, though sometimes referred to as ‘Master Standard Frequency’ it is not intended as an acronym according to National Physical Laboratory.

MSF Time Server

Here is a link to the MSF Website.

It is delivered with a MSF outdoor antenna that can be connected with standard coaxial cable with up to 300m length.

GPS Zeitserver

Durch die spezielle Antennen Up/Down Converter Technik ist es möglich die GPS Antenne mit Standard Koaxialkabel z.B. RG58 bis zu 300m mit RG213 bis zu 600m abzusetzen.

Zusätzlich besteht die Möglichkeit die Antenne über eine Fiber Optic Verbindung störungssicher bis zu 2km abzusetzen. GPS Fiber Optic Antennen Verbindung.

Das GNSS-Systeme arbeitet mit Satelliten, die  mehrere tausend von  Meilen über der Erdoberfläche verwended werden und diese die Informationen an die Erd GPS Geraet, wie z.B im Auto senden. Es ist dieses Signal, das von dem GPS Geraet genutzt wird, um die genaue Position zu ermitteln. Dieses kann nur durch die sehr genaue Atomuh, die in dem Satelliten eingebaut ist passieren. Bei einer Ungenauigkeit von soagr 1 Sekunde, kann es fatale Auswirken auf die Distanz haben.

Durch diese genauen Zeitquellen, können GPS und die neue Erschaffung von GNSS Systemen,  ein Absolutes oder UTC (Koordinierte Universale Zeit) Zeitquelle benutzten. Diese Zeitquelle kann von alle Computernetze verwendet werden, die einen NTP Server (Netzzeitprotokoll) führen, um alle Maschinen und Geräte zu derselben Zeit zu synchronisieren.

NTP ist ein Protokoll, das dafür entworfen ist, Computer und Netzgeräte zu einer externen Zeitverweisung zu synchronisieren.

GPS ist eine ideale Zeit und Frequenzverweisung, weil es hohe Zeitgenauigkeit in der ganzen Welt zur erfuegung stellen kann, zur relativ guenstigen Preisen. Jeder GPS Satellit übersendet in zwei Frequenzen L2 für den militärischen Gebrauch und L1 für den  Buergerlichen Gebrauch. Preisgünstige GPS Antennen und Empfängern sind jetzt weit verfügbarer und hingebungsvoller.Die  NTP GPS Server haben einen relativ niedrigen Kostpreis.

Das, durch den Satelliten übersandte Radiosignal, kann durch ein Fenster emfangen werden, doch von hohen Gebaeuden blockiert werden. Die Ideale Podition fuer die GPS Antenna ist auf dem Dach eines Hauses oder Gebaeudes. Jedoch kann auch hier, die ANtenne vom Gwitter erfasst werde und somit ein Entstoergerat sehr empfehlenswert ist.

Ein NTP GPS Server ist ideal , um NTP Zeitserver oder alleinstehenden Computer, die genaue Zeit zu vermitteln. Sogar mit der relativ niedrigen Kostenausrüstung, kann die Genauigkeit von Hundert Nanosekunden (eine Nanosekunde = ein Milliardstel einer Sekunde) erreicht werden.

Connects directly to Ethernet, fits in a standard 19" rack, incorporates a Galleon GPS Computer Clock, tracks up to 12 satellites, with remote antenna that can be positioned up to 1,000m (3,000 ft) away with LCD signal strength display for easy installation, an integral time display and Ethernet CAT5 connection. Software driver for uni-cast, broadcast or multi-cast time synchronization. Conforms to Network Time Protocol Version 4 (NTP V4).

TS-700-GPS GPS Clock

The GPS clock supplies time to the computer via a serial interface (USB optional) and the software driver Time-Sync NTP will synchronize the time on a Windows NT, 2000, or 2003 Server or a Windows XP workstation and configures as a NTP time server to provide accurate time to PC's, workstations and other servers across the network. This software conforms to the Network Time Protocol (NTP) and can be set up as a Stratum 1 Time Server, Time-Sync runs as a service and configures as a NTP Timeserver so can synchronize UNIX machines and any equipment that conforms to NTP.

There are three methods that time servers can receive the UTC signal from. Firstly, the internet, although unsecured and without any guarantees of accuracy Internet time references are only suitable for networks where precision and security are not a concern.

The second method is via the GPS network which is available everywhere but only where an antenna has a clear view of the sky.

The third and simplest method to receive an accurate and secure UTC time reference is to use the national time and frequency broadcasts. In the UK the National Physical Laboratory transmits the British signal from Anthorn in Cumbria. This signal, known as MSF, can be received in most places in the UK although local topography can interfere with the transmission.

To receive the MSF broadcast a radio referenced NTP time server is required. This will receive the radio transmission via 65 kHz in long wave and distribute it amongst the network.
A radio based NTP server usually consists of a rack-mountable time server, and an antenna, consisting of a ferrite bar inside a plastic enclosure, which receives the radio time and frequency broadcast. The antenna should always be mounted horizontally at a right angle toward the transmission for optimum signal strength.

Similar national timing transmissions are broadcast from other countries in the US the signal is referred to as WWVB and is broadcast by the NIST (National Institute for Standards and Technology) in Fort Collins, Colorado, other systems are broadcast in Frankfurt, Germany (DCF-77), Japan (JJY) and France ( TDF).

Hier ein kleines Beispiel meiner Photoshop Fähigkeiten:

Das obige Foto zeigt Meinbergs PTP und NTP tauglichen 19 Zoll Rackeinbau NTP Zeitserver mit GPS Synchronisation.

The name of this model is Lantime M100 and it comes with a GPS timing receiver, antenna, and 20 m of cable but without display and for DIN rail mount, because of that i think it is mainly for industrial environments, like substations or similar. Here is a picture from the Meinberg website:

NTP (Network Time Protocol) is a protocol already installed on Windows XP and Windows Time uses it to keep machines synchronised to the single time source. There are several timing sources available on the Internet but Microsoft and others strongly recommend that you configure a time server with a hardware source rather than from the Internet where there is no authentication.

Specialist NTP time servers are available that can receive a reliable time source via the GPS signal or specialist radio transmissions that get their time from atomic clocks.

If you wish to configure Windows XP to operate as a time server then first thing is to locate the Windows Time subkey. To do this:

Run Regedit (Click start/run/then type REGEDIT/and click enter.

Note: editing your system registry can cause problems with your system. It is advisable to back up your system before editing the registry.

Now locate the following subkey: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\W32Time\parameters\

Right click the right-hand side and click Modify. In the Edit Value box, under Value Data, type NTP and then click OK.

Now go to the Config folder and right-click AnnounceFlags, Modify and in the Edit DWORD Value box, under Value Data, type 5, and then click OK.

Locate this subkey:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\W32Time\TimeProviders\NtpClient\

Right-click in the right-side window and Modify. Edit the DWORD value box and type the number of seconds you want for each poll under Value data, i.e.: 900 will equal 15 minutes. The poll field represents the polling interval between NTP poll packets.

To enable the NTP server locate the subkey: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\W32Time\TimeProviders\NtpServer\

Right click enabled (in the right-hand window) then Modify. Edit the DWORD Value and type 1. Right-click NtpServer, then Modify and in the Edit DWORD Value under Value Data type Peers, then click OK.

Locate: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\W32Time\config

In the right pane, right-click MaxPosPhaseCorrection, then Modify, in the Edit DWORD Value box, under Base, click Decimal, under Value Data, type a time in seconds such as 3600 (an hour) then click OK. This adjusts the connection settings.

Now go back and click:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\W32Time\config

In the right pane, right-click MaxNegPhaseCorrection, then Modify.

In the Edit DWORD box under base, click Decimal, under value data type the time in seconds you want to poll such as 3600 (an hour).

Exit Registry then restart windows time service by clicking Start/Run then typing:
net stop w32time && net start w32time.
on each computer, other than the domain controller, type: W32tm/resync/rediscover.
The time server should be now up and running.

All PC’s and networking devices use clocks to maintain an internal system time. These clocks, called Real Time Clock chips (RTC) provide time and date information. The chips are battery backed so that even during power outages, they can maintain time.

However, personal computers are not designed to be perfect clocks, their design has been optimized for mass production and low-cost rather than maintaining accurate time. However, these internal clocks are prone to drift and although for many application this can be quite adequate, often machines need to work together on a network and if the computers drift at different rates the computers will become out of sync with each other and problems can arise particularly with time sensitive transactions.

For some transactions it is necessary for computers to be perfectly synchronised, even a few seconds difference between machines can have serious effects, such as finding an airline ticket you had booked had been sold moments later to another customer or you could draw your savings out of a cash machine and when your account is empty you could quickly going to another machine and withdraw it all again.

Time servers are like other computer servers in the sense they are usually located on a network. A time server gathers timing information, usually from an external hardware source and then synchronises the network to that time.

Most time servers use NTP (Network Time Protocol) which is one of the Internet’s oldest protocols still used, invented by Dr David Mills from the University of Delaware, it has been in utilized since 1985. NTP is a protocol designed to synchronize the clocks on computers and networks across the Internet or Local Area Networks (LANs).

NTP utilises an external timing reference and then synchronises all devices on the network to that time.

Often time servers are synchronised to a UTC (Coordinated Universal time) source which is the global standard time scale and allows computers all over the world to synchronised to exactly the same time. This has obvious importance in industries where exact timing is crucial such as the stock exchange or airline industry.

There are various sources that a time server can use as a timing reference. The Internet is an obvious source, however, internet timing references from the Internet such as nist.gov and windows.time can not be authenticated, leaving the time server and therefore the network vulnerable to security threats.

In a modern global economy time synchronisation is essential for carrying out time sensitive transactions such as booking an airline ticket to bidding on an Internet auction site. If clocks were not synchronised to the same time you may find your airline seat sold after you had bought it and Ebay’s administrators would not be able to discover whose bid was the latest.

NTP is a multi-tiered system, each tier being called a stratum. Servers at each tier communicate with each other (peer) and provide time to lower strata. Servers at the top stratum, stratum 1 connect to an atomic clock either over the Internet or by a radio or GPS receiver while a stratum 2 server will connect to a stratum 1.

NTP uses an algorithm (Marzullo’s algorithm) to synchronise time on a network using time scales like UTC (Coordinated Universal Time or Temps Universel Coordonné) and can support such features as leap seconds - added to compensate for the slowing of the Earth’s rotation.

NTP (version 4 being the latest) can maintain time over the public Internet to within 10 milliseconds (1/100th of a second) and can perform even better over LANs with accuracies of 200 microseconds (1/5000th of a second) under ideal conditions.

NTP time servers work within the TCP/IP suite and rely on UDP (User Datagram Protocol). A less complex form of NTP called Simple Network Time Protocol (SNTP) that does not require the storing of information about previous communications, needed by NTP, is used in some devices and applications where high accuracy timing is not as important and is also included as standard in Windows software (although more recent versions of Microsoft Windows have the full NTP installed and the source code is free and readily available on the Internet).

The NTP program (known as a daemon on UNIX and a service on Windows) runs in the background and refuses to believe the time it is told until several exchanges have taken place, each passing a set of tests. If the replies from a server satisfy these ‘protocol specifications’, the server is accepted. It usually takes about five good samples (five minutes) until a NTP server is accepted as a source for synchronisation.

Synchronisation with NTP is relatively simple, it synchronises time with reference to a reliable clock source such as an atomic clock, although these are extremely expensive and are generally only to be found in large-scale physics laboratories, however NTP can use either the Global Positioning system (GPS) network or specialist radio transmission to receive UTC time from these clocks.

A simplified version of NTP called Simple Network Time Protocol (SNTP) exists that does not require the storing of information about previous communications as required by NTP. It is used in some devices and applications where high accuracy timing is not as important and is installed on older versions of Microsoft Windows. Windows since 2000 has included the Windows Time Service (w32time.exe) which uses SNTP to synchronise the computer clock. NTP is also available on UNIX and LINUX (download via NTP.org).

The good news: Once you have the serial cable, using the GPSr for ntpd is a matter of seconds.

The Garmin must be instructed to spew out location data on the serial port, which can be accomplished through the interface configuration menu. The default of "GARMIN" for the serial port has to be changed to "NMEA-In / NMEA-Out". After making this setting, you can use Minicom to connect to the serial port at 4800/8N1 where you will see a constant stream of data. The $GPRMC lines contain the information that is required for NTP. (Click here for details about the format.)

According to the ntpd documentation, the Garmin will be configured as a "generic NMEA GPS receiver".

ntpd will require a symlink in /dev so it knows where to find the GPSr. In my case, the Garmin is connected to /dev/ttyS0. Hence, the symlink needs to be created as follows:

# ln -s ttyS0 /dev/gps0

ntpd accesses this device through a pseudo IP address that will be used in ntp.conf:

server 127.127.20.0

Behold the NMEA peer:

ntpdc> peers
     remote           local      st poll reach  delay   offset    disp
=======================================================================
*GPS_NMEA(0)     127.0.0.1        0   64  377 0.00000  0.001443 0.03511

Be advised that it will take a few minutes until ntpd has synchronized with the GPSr. If you can't get your NTP clients to synchronize with your NTP server, leave it alone for a while and try again later. Synchronization with the GPSr is complete as soon as the output from "peers" no longer starts with "=" but with an asterisk (as above). I learned this the hard way during Y2K testing in 1999 when an NTP server just wouldn't synchronize. I restarted it over and over again. At the end of my tether, I went out for lunch and left the defunct server behind. When I came back, everything had just fallen into place, the clock was synchronized and so were the clients. :-)

Computer networking is one of the most difficult aspects of information and communications technology (ICT). The logistics of connecting terminals, routers, printers and all the other devices can leave many administrators with a constant headache.
One of the most important aspects that often gets overlooked and can have disastrous consequences is that of time synchronization.

It is imperative that all devices on a network are telling the same time as timestamps, the format a computer relays time to each other, are the only form of reference a computer can use to establish a sequence of events. If different machines on a network are telling different times then unforeseen consequences such as emails arriving before they have technically been sent and other anomalies will make the administrator’s headache even worse.

What’s more a computer network that is not synchronized is open to security threats and even fraud. Fortunately the NTP time server has been around for many years and can ease the headache of time synchronization .

NTP (Network Time Protocol) is one of the oldest protocols used by computer networks. Developed nearly three decades ago NTP is a protocol that checks the time on all devices on network and adds or subtracts enough time to ensure they are all synchronized.

NTP requires a time reference to synchronise the network’s clocks to. Whilst NTP can synchronize a network to any time an authoritative time source is obviously the best solution. UTC (Coordinated Universal Time) is a globally used timescale based on the time told by atomic clocks. As atomic clocks lose less than a second of time in over a thousand years, UTC is by far the best timing source to synchronize a network to. Not only will your network be perfectly synchronized together but also your network will be synchronized to the same time as millions of computer networks all from around the world.

A NTP server can receive a UTC time reference from several sources. The Internet is the most obvious source, however Internet timing sources are notoriously inaccurate and those that are not can be relatively useless if the distance is too far away. Also having placed your NTP server securely behind your firewall it does seem pointless to have to keep a hole open in it to allow the NTP server to poll the timing reference from across the web and leave the entire network vulnerable, particularly as NTP authentication (NTP’s own security measure) is not possible over the Internet.

There are two far more secure and accurate methods of receiving a UTC timing reference. The first is to utilise the national time and frequency transmissions that several countries broadcast from their national physics laboratories. These are usually broadcast via long wave which has an advantage of being able to be picked up inside a server room although many countries do not have such a signal.

However, many NTP servers can utilize the timing signal broadcast by the onboard atomic clocks of the GPS (Global Positioning System) satellites.  This signal is available everywhere but a GPS antenna is required that can get a clear view of the sky.

By utilizing a UTC timing source either through the GPS network of radio transmission a computer network can be synchronized to within a few milliseconds of UTC time.

For computers a time stamp is the only point of a reference a computer can use to establish a sequence of events. Timestamps are used in everything from sending an email to debugging a system and ensuring your network is secure.

Nearly all time servers rely on NTP (Network Time Protocol) to synchronise devices once a timing source is selected Whilst it is not the only protocol designed for synchronisation it is by far the most widely used, primarily because it has been under constant development since its inception nearly three decades ago.

A NTP server works by receiving a time reference from an authoritative source, normally a UTC time source. UTC (Coordinated Universal Time) is an International timescale based on the time told by atomic clocks.

Unfortunately many administrators opt to use an Internet timing source to run their time server with. However, several studies of Internet timing references discovered that nearly two thirds were inaccurate by over several seconds (almost an eternity in the world of NTP) and those that weren't were often too far away from a client to provide reliable accuracy.

Probably the most accurate and widely used method of retrieving a reliable time reference is to use a dedicated GPS Time Server, many national physics laboratories also broadcast the time told by their atomic clocks via long wave radio. These signals can be received by a time server if it is equipped with a radio receiver although not every country broadcasts one and the transmissions are susceptible to interference and local topography.

Once this secure UTC time reference is received the time server can then distribute the time across the network. The timestamp itself is represented as a single ascending number. The number is the amount of seconds from a prime epoch (start number) normally 01 January 1900. The time server checks the system clocks on its clients and advances or retracts the time until all machines on the network are synchronised.

When a communication is sent from one computer and received by seems to have arrived before it was sent and this can cause unforeseen problems.

Think about buying an airline ticket only to turn up at the airport to find somebody has bought your seat after you did because they booked it on a computer with a slower clock!

Dr David Mills of the University of Delaware, realised the need for a synchronisation tool and developed Network Time Protocol (NTP). Although not the first and only time synchronisation protocol it is by far the most widely used and probably owes its success, to the now Professor Mills, and his dedicated team for its continual development.

What started out as a protocol that could manage synchronisation to a few milliseconds is now able to keep computer clocks to within a few nanoseconds of each other (milli = 1/thousandth nano= 1/billionth).

NTP is hierarchical and is divided into Strata. A clock source is referred to as stratum 0, whilst a NTP time server is stratum 1, Computers and devices that receive a time from a stratum 1 server become stratum 2 and so on.

This hierarchical structure means that tens of thousands of devices can synchronise to the same time whilst not inundating the NTP time server or the bandwidth of the network.

NTP time servers rely on using a reference clock. whilst this can be anything from a wrist watch or a computer's system clock, it would be pointless to use a reference clock that was not accurate in itself.

Most networks that use a NTP time server will use a UTC time source. UTC or Coordinated Universal Time is based on the time told by the incredibly accurate and expensive atomic clocks. An atomic clock's time signals can be picked up either from across the Internet (although accuracy varies and is dependent on distance), specialist radio transmissions broadcast in several countries (including the US, UK, Germany, France and Japan) or via the American GPS (Global Positioning System).

The NTP time servers then distribute this stratum 0 time source between all devices on a network that connects with the NTP time server. NTP will either then advance or hold the system clock to match with the timing reference.

NTP also has in-built security measures called authentication. these are a set of keys that are encrypted and used to identify both client and server. Unfortunately authentication is unavailable on Internet time references which is why both Microsoft and Novell suggest hardware sources should be used as a timing reference.

NTP is currently on version 4 with version 5 under development and is open source and freely available to download via ntp.org.

NTP works on a hierarchical basis each hierarchy is referred to as a strata. A stratum 0 server is an absolute time source such as an atomic clock, a stratum 1 server is the NTP time server that receives this time signal and server's that get a time signal from a stratum 1 server are referred to as stratum 2 (and so up to 16 strata can be supported).

Arranging servers into a hierarchy like this prevents too much traffic from trying to communicate withe NTP time server allowing networks of hundreds and even thousands of devices to be synchronised.

It is good practice to synchronise the lower strata to multiple servers as this ensures greater accuracy.

A dedicated NTP time server should get its timing information from a stratum 0 source. The most accurate and secure way of doing this is by using the time signal from the GPS network.

A GPS antenna should be connected to the NTP time server and the antenna must be mounted on the roof to ensure a clear view of the sky (the satellite signals work by line-of-sight). This of course can be impractical (especially if your server room is on the ground floor of a skyscraper).

An alternative is to use the specialist national time and frequency transmissions that are broadcast from certain countries. The USA, UK, France Germany and Japan all have their own signals. A radio receiver can pick up these transmissions even indoors although the accuracy is slightly reduced to that of the GPS system.

A dedicated NTP time server using GPS or a radio transmission can provide accuracy to within a few milliseconds of a timing source.

Not every country transmits a national time and frequency broadcast which means if a time server is to be located outside of the US, Japan, Germany, the UK or France it is doubtful that a signal  will be received (although some of these broadcasts can be received in neighbouring states).

The radio frequency is also susceptible to atmospheric interference and can be blocked by local topography.

GPS on the other hand can be received anywhere in the world it is also slightly  more accurate - a typical GPS receiver can provide timing information to within a few nanoseconds of UTC (Coordinated Universal Time) while national time and frequency broadcasts are accurate to 1 - 20 milliseconds.

However, using a radio broadcast as a timing reference is that it is possible to receive it from inside a building (although basements and rooms with metal furnishings can cause interference). The drawback of GPS is that the antenna needs to be situated on the roof to have a clear view of the sky to be able to find and receive the signal broadcast from the satellites (although it is possible sometimes to receive a signal through window).

There is no master clock that the world relies on but there is a global timescale called UTC (Coordinated Universal Time). UTC was developed after the development of atomic clocks. It is based on a combination of International Atomic Time (TAI - the time told by these clocks) and Greenwich Meantime (GMT).

While TAI is incredibly accurate it is actually too accurate as the Earths rotation slows due to the effect of the moon's gravity. It compensates for this by adding Leap Seconds to keep in similar to GMT, otherwise night would creep into day.

UTC is governed by a constellation of atomic clocks all over the world to ensure even more accuracy (and to prevent any political squabbles).

A NTP Time Server is a device that can receive UTC time through either a radio transmission (in certain countries) or the GPS network. NTP (Network Time Protocol) is designed to synchronise all devices on a network to the time received by the NTP time server.

A NTP time server allows networks all over the world to communciate safely synchronised to the exact same time.

UTC and NTP time servers allow the whole world to communicate within the same timescale. Without them, many of the applications and processes we do online would be impossible such as Internet trading, the stock exchange, buying airline tickets and even sending and receiving email.