Understanding Computer Timestamps with NTP

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The way a computer deals with time is totally different to the ways humans perceive it. We arrange time into seconds, minutes, hours, days, weeks, months and years, while computers on the other hand arrange time as a single number representing the seconds that have passed from a single point in time, known as the prime epoch.

Most computers use NTP (Network Time Protocol) to deal with time and on networks many are synchronised using a dedicated NTP time server.  NTP knows nothing about days, years or centuries, only the seconds from the prime epoch.  This prime epoch is set (for most systems) at midnight at the turn of the century twentieth century that for a human would be recorded as something like: 00:00 – 01,01,1900.

Computers, however, count time as the number of seconds past this point. If a computer was around in 1900 its timestamp on midnight January 1 would be 0 while in 1972 at the same date the timestamp would be 2,272,060,800, which represents the number of seconds since 1900.

The timestamps restart every 136 years with the next wrap around due in 2036, this has caused uneasiness amongst some who fear a Millennium Bug type scenario, although most doubt such events would occur, however, when a wrap-around of the timestamp does happen an era integer will be added (+1), to allow computers to deal with time spans that cover more than one wrap-around.  If computers and NTP need to deal with time that spans before the prime epoch a negative integer is used (for the year 1500 a -3 will be used to represent three cycles of 136 years).

Timestamps are used in virtually every transaction that modern computers are tasked to do such as sending emails, debugging and programming. Because time is linear, a computer knows that each timestamp is always greater than the previous one and therefore computers and NTP find it difficult to deal with inaccuracies in time, particularly when time suddenly appears to go backwards.

This can happen if computers are not synchronised to the same time. If an email is sent to a machine with a slower clock, it appears to the computer to have been received before it has been sent.  Lack of synchronisation can serious problems and can even leave a system vulnerable to malicious attacks and even fraud.

Because of this, most computer networks are synchronised to UTC (Coordinated Universal Time). UTC is a global timescale and the same for everybody worldwide it is based on the time told by atomic clocks which are highly accurate, neither gaining nor losing a second in millions of years.

Most computer networks use a dedicated NTP time server to receive a UTC time to synchronise their computers too.  UTC is available from across the Internet (although unsecured), via the GPS network (Global Positioning System), or by receiving national time and frequency broadcasts via long wave.

NTP synchronises a computer by checking the received UTC time and adding to or holding a computer’s timestamp until it perfectly matches UTC. By using a dedicated NTP time server UTC can be maintained on a network to a few milliseconds of UTC time.

Network Time Server Keeping Control of Time

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Computer networking can seem an intimidating undertaking. However, a computer network is really just a number of machines connected together for ease of data transfer and security. They can be very small such as two computers in a home network to really large networks consisting of hundreds and thousands of machines.

When a computer or device is connected to a network then there is only one point of reference that the computers can use to establish the order of events and applications and that is time.

Time, in the form of time stamps are used by most applications and this is when problems in computer networks can occur.

Computers tell the time by using a software clock. This is based on a system clock that keeps time when the computer is off. However, computers internal clocks are wholly inaccurate. They tend to drift up to several seconds a week. On a network when there is more than one machine, this can cause severe problems if the machines are drifting at different rates.

Emails may arrive before they have been sent and the whole network can be vulnerable to security threats and even fraud!

A network time server is used to synchronize a computer network to a single time source. This time source can be anything from an internal clock on a computer to the time told by a wrist watch. However, to ensure perfect accuracy and to keep a network synchronized to the rest of the world then a UTC time source should be used.

UTC (Coordinated Universal Time) is a global timescale based on the time told by atomic clocks. A network time server can receive a UTC time source from across the Internet (although unsecured), via the GPS (global positioning system) network or via specialist radio transmission from national physics laboratories.

Most network time servers use NTP (Network Time Protocol) to distribute the timing reference throughout the network. NTP is not the only timing protocol designed to do this although it is, however, by far the most widely used.

GPS Time Server and NTP (Network Time Protocol)?

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We are all used to Satellite Navigation by now. More and more people are installing those little black boxes into their cars and throwing away their old paper road maps. The advantages of satellite navigation are many fold – from constant updates keeping the maps current to being able to pin point your location miles from any landmarks or road signs but GPS has more uses than merely triangulating a position for direction finding, it can be utilized to provide time and frequency information worldwide.

Since the early 1990’s the Global Positioning system (GPS) has been the worlds’ only fully functioning Global Navigational Satellite System (GNSS). Run by the American military, GPS (sometimes referred to as NAVSTAR) has allowed accurate timing and location finding all over the world.

To accurately pinpoint a location, all GNSS systems require an absolute time source, that is a time source as accurate as humanely possible such as that from an atomic clock. Without knowing exactly what the time is a GNSS satellite would not be able to accurately pin point a location (as the Earth, satellites and people are all moving about a location can only be defined by a position and time). Because of the distance of the satellites away from the Earth, even an inaccuracy of a second or two could mean a sat nav’s location could be miles out.

For this reason each satellite has a highly accurate atomic clock onboard which can also be used by NTP (Network Time Protocol) servers to synchronise computer networks. GPS is an ideal time and frequency source because it can provide highly accurate time anywhere in the world using relatively cheap components.

A GPS receiver decodes the signal sent from the GPS antenna to a computer readable protocol which can be utilised by most time servers and operating systems including, Windows, LINUX and UNIX.

The GPS receiver also outputs a precise pulse every second that GPS NTP servers and computer time servers may utilise to provide ultra-precise timing. The pulse-per-second timing on most receivers is accurate to within 0.001 of a second of UTC (Coordinated Universal Time or Temps Universel Coordonné).

GPS is ideal in providing NTP time servers or stand-alone computers with a highly accurate external reference for synchronisation. Even with relatively low cost equipment, accuracy of hundred nanoseconds (a nanosecond = a billionth of a second) can be reasonably achieved using GPS as an external reference.

In 2002, the European Space Agency and European Union agreed to build Europe’s own GNSS called Galileo. To compete with the new and more advanced GNSS technologies the GPS programme is currently being upgraded and it is expected that when Galileo begins relaying signals both systems will become interoperable allowing even more accuracy in timing and positioning.

Choosing a dedicated GPS Time Server

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Because of the advancement in satellite navigation technologies and the increased coverage of the American GPS satellite network, many more administrators are choosing GPS as a source for a timing reference to synchronize their time servers too.

Traditionally many more people opted to receive a timing source from either across the Internet or via specialist national time and frequency transmissions.  However, the Global Positioning System is now by far the most popular method to receive a UTC time source from.

UTC (Coordinated Universal Time) is the global timescale based on the time told by atomic clocks which are the most accurate of time keeping devices.

A GPS time server is a relatively simple piece of hardware. Normally it consists of a dedicated NTP server with software, a GPS receiver and a GPS antenna. The antenna is the only drawback in using a dedicated GPS time server as it has to be positioned on the roof to have a clear view of the sky, although some dedicated GPS time servers can still keep time synchronized if they only receive a signal for a few hours a day, although this is not the most accurate way of time synchronization.

Once connected, the GPS time server will receive the time signal from the GPS satellites and then distribute it to all devices that require synchronization.

Most time servers, whether they receive a GPS signal or not, will use Network Time Protocol (NTP) to distribute the time signal to all devices.

NTP is one of the Internet’s oldest protocols and is by far the most widespread time synchronization protocols used. NTP is under constant development and can accurately synchronise a network to within a few milliseconds of UTC time via a dedicated GPS time server.

Configuring a NTP Time Server using Windows XP

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Microsoft Windows XP has a time synchronisation utility built into the operating system called Windows Time (w32time.exe) which can be configured to operate as a network time server. It can be configured to both synchronise a network using the internal clock or an external time source.

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:

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:

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.

Configuring a Network Time Server on Windows

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All versions of Windows Server since 2000 have included a time synchronization facility, called Windows Time Service (w32time.exe), built into the operating system. This can be configured to operate as a network time server synchronizing all machines to a specific time source.

Windows Time Service uses a version of NTP (Network Time Protocol), normally a simplified version, of the Internet protocol which is designed to synchronise machines on a network, NTP is also the standard for which most computer networks across the global use to synchronise with.

Choosing the correct time source is vitally important. Most networks are synchronized to UTC (Coordinated Universal Time) source. UTC is a global standardized time based on atomic clocks which are the most accurate time sources.

UTC can be obtained over the Internet from such places as time.nist.gov (us Naval Observatory) or time.windows.com (Microsoft) but it must be noted that internet time sources can not be authenticated which can leave a system open to abuse and Microsoft and others advise using an external hardware source as a reference clock such as a specialized network time server.

Network time servers receive their time source from either a specialist radio transmission from national physics laboratories which broadcast UTC time taken from an atomic clock source or by the GPS network which also relays UTC as a consequence of needing it to pin point locations.

NTP can maintain time over the public Internet to within 1/100th of a second (10 milliseconds) and can perform even better over LANs.

To configure Windows Time Service to use an external time source simply follow these instructions.

Locate the registry subkey.


Right click type then Modify the Value by inserting NTP in the Value Data box.

Right click ReliableTimeSource, then Modify the Edit DWORD Value box, by inserting 0 (zero).

Right-click NtpServer then Modify the Edit Value by typing the Domain Name System (DNS), (note each DNS must be unique).

Now locate the original subkey and right-click Period Modify the Edit DWORD Value box with the poll interval (how often a NTP server polls the time), under Value Data (recommended 24)

Run the following command line, Net stop w32time && net start w32time.

Now enable NTP by locating the subkey, HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\W32Time\TimeProviders\NtpServer\

Right click Enabled and modify the Value data box by typing 1.

Right Click SpecialPollInterval in the right pain of the subkey HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\W32Time\TimeProviders\NtpClient\SpecialPollInterval. Edit the DWORD value box the time you want for each poll (900 will poll every 15 minutes)

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

To configure the time correction settings right click MaxPosPhaseCorrection, then modify the DWORD Value box with a time in seconds such (select decimal under base first, 3600 = one hour)

Now do the same for MaxNegPhaseCorrection the restart windows time service by running (or alternatively use the command prompt facility) net stop w32time && net start w32time.

To synchronise each machine simply type W32tm/ -s in the command prompt and the time server should now be working correctly (note it may take several polls before the correct time is displayed).

Using WWVB as a Timing Reference for NTP Servers

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Atomic clocks are incredibly expensive and generally they are normally only to be found in large scale physics laboratories such as MIT (Massachusetts Institute of Technology), NIST (National Institute of Standards and Technology (Colorado) or the National Physical Laboratory in the UK.

Fortunately many national laboratories broadcast the UTC (Coordinated Universal Time) time from their atomic clocks via a radio transmission.

In the US the national timing broadcast is called WWVB and is broadcast by NIST (National Institute fro Standards and Time) in Fort Collins, Colorado. The WWVB broadcast is used by millions of people throughout North America to synchronize consumer electronic products like wall clocks, clock radios, and wristwatches. In addition, WWVB is used for high-level applications such as network time synchronization utilizing NTP.

The time code contains the year, day of year, hour, minute, second, and flags that indicate the status of Daylight Saving Time, leap years, and leap seconds.

WWVB broadcasts on 2.5, 5, 10, 15, and 20 MHz and for most users in the United States, the received accuracy should be less than 10 milliseconds (1/100 of a second).

While many NTP servers now use GPS to receive a timing reference, the advantage of using a radio transmission is that a signal can be received indoors (a GPS antenna needs a good view of the sky).

However, the radio signal has a finite range and can be blocked by skyscrapers, mountains and dense conurbations. 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 UK the signal is referred to as MSF and is broadcast by the National Physical Laboratory in Cumbria, other systems are broadcast in Frankfurt, Germany (DCF-77), Japan (JJY) and France (TDF)

Using National Time and Frequency Signals as a NTP Timing Reference

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This article explores how to use national time and frequency radio transmissions for network time synchronisation.

The importance of an authenticated timing reference to synchronise a computer network to, cannot be stressed highly enough.

While there are hundreds and quite possibly thousands of internet based timing sources these can’t be authenticated leaving a system open to viruses, malicious hackers or malware.

Furthermore, a survey by MIT (Massachusetts Institute of Technology) found that nearly half of internet timing sources were offset by over ten seconds and only a third could be regarded as being ‘useful ’ also it was discovered that many were too far away from peers to provide any useful accuracy.

Most dedicated network time servers are designed to receive a timing signal from the GPS (Global Positioning System), primarily because it is the most accurate and can be received from anywhere on the globe.

However, there are situations where it may not be practical to use a GPS time server. A GPS antenna has to be situated on a rooftop and have a clear view of the sky which may prove difficult if the server is on the ground floor of a multi-storey sky-scraper. Many administrators also dislike the hassle and expense of having to run a cable up a building and install an antenna or if there are possibilities the server room maybe relocated and the process has to be repeated.

Fortunately many countries’ national physics laboratories broadcast a time and frequency signal from a radio transmitter. In the US the signal is referred to as WWVB and is broadcast by NIST (National Institute for Standards and Technology) in Colorado. In the UK the National Physical Laboratory (NPL) broadcasts the MSF signal from Cumbria and similar systems are broadcast in Germany (DCF-77), Japan (JJY) and France (TDF).

Unfortunately not every country transmits a national time and frequency broadcast so if a time server is to be located outside of the US, Germany, UK, France or Japan it may be doubtful that a signal could be received (although many of the these transmissions can be received in neighbouring countries).

Radio signals are also easily susceptible to atmospheric interference and can be blocked by mountains, sky-scrapers or other topography. However, an upside to using a radio receiver is that it will receive a signal inside a building.

While a radio transmission is not as accurate as a GPS time signal a dedicated network time server receiving a radio signal can still provide accuracy between 1 – 20 milliseconds (a millisecond is 1/1000 of a second) which is more than adequate for the needs of network synchronisation.

How to Build and Configure Your Own Radio Referenced NTP Time Server

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Many people are probably familiar with the Internet Time tab when setting their clock in Windows. This is a basic form of NTP (Network Time Protocol) called SNTP (Simple Network Time Protocol) that polls a NTP server every so often to synchronize time to. However, full NTP does a lot more, such as polling several servers to determine what is the best and most stable times source.

Creating your own NTP time server is relatively straightforward and if you are a resident of the US then the simplest way is to use a relatively cheap receiver module, set to receive the WWVB timing signal which is broadcast by NIST (National Institute of Standards and Time) at a frequency of 60 kHz.

To create your own product using the WWVB time signal, WWVB receiver modules, are readily available at low cost. When looking for a receiver module there are several points to consider:

There should be a simple interface to allow easy integration of accurate timekeeping into electronic equipment etc. The receiver should use dedicated chips designed specifically to receive the WWVB (USA) time signal. The advantage of these over other solutions is that the modules include the necessary support electronics, tuned crystals, etc and are preassembled, tested and are usually in miniature form.

When combined with a suitable Antenna the receiver module acts as a complete time code receiver, providing a serial digital data output stream for external decoding.

Once assembled and tested so the module is receiving a signal it can then be connected to your computer or server.

Once connected the time server should be configured using the registry editor. Simply follow these steps:

Locate the following subkey: HKEY_LOCAL_MACHINESYSTEMCurrentControlSetServicesW32TimeParametersType
In the right pane, right-click Type then click Modify, in edit Value type NTP in the Value data box then click OK.

Locate the following subkey: HKEY_LOCAL_MACHINESYSTEMCurrentControlSetServicesW32TimeConfigAnnounceFlags.

In the right pane, right-click AnnounceFlags and click Modify. The ‘AnnounceFlags’ registry entry indicates whether the server is a trusted time reference, 5 indicates a trusted source so in the Edit DWORD Value box, under Value Data, type 5, then click OK.

To enable the Network Time Protocol; NTPserver, locate and click: HKEY_LOCAL_MACHINESYSTEMCurrentControlSetServicesW32TimeTimeProvidersNtpServer

In the right pane, right-click Enabled, then click Modify. In the Edit DWord Value box, type 1 under Value data, then click OK.

Now go back and click on: HKEY_LOCAL_MACHINESYSTEMCurrentControlSetServicesW32TimeParametersNtpServer

In the right pane, right-click NtpServer, then Modify, in the Edit DWORD Value under Value Data type In the right pane, right-click NtpServer, then Modify, in the Edit DWORD Value under Value Data type the Domain Name System (DNS), each DNS must be unique and you must append 0x1 to the end of each DNS name otherwise changes will not take effect.

Now click Ok then locate and click the following: HKEY_LOCAL_MACHINESYSTEMCurrentControlSetServicesW32TimeTimeProvidersNtpClientSpecialPollInterval

In the right pane, right-click SpecialPollInterval, then click Modify. In the Edit DWORD Value box, under Value Data, type the number of seconds you want for each poll, ie 900 will poll every 15 minutes, then click OK.

To configure the time correction settings, locate: HKEY_LOCAL_MACHINESYSTEMCurrentControlSetServicesW32Timeconfig

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.

Now go back and click: HKEY_LOCAL_MACHINESYSTEMCurrentControlSetServicesW32Timeconfig 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 (polls in one hour)Exit Registry Editor

Now, to restart windows time service, click Start, Run (or alternatively use the command prompt facility) and type:

net stop w32time && net start w32time. And that’s it your time server should be now up and running.

Atomic Clocks and NTP Servers

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This article explains the origins and workings of atomic clocks and how they are used to synchronise computer networks all over the world using NTP servers.

In conventional electronic clocks time is kept by running an electrical current through an oscillator which produces a repetitive electrical signal this is then governed by a quartz crystal to keep precision. These crystal oscillators are far more accurate than mechanical clocks but will still drift, perhaps over a second a week.

For day-to-day use crystal oscillators are a fine way to keep track of time; in the everyday running of our lives, a second makes very little difference, however, as light or radio waves can travel 300,000 miles in a second, some high technologies such as satellite navigation or global communication, require far more accuracy to be possible.

Atomic clocks are a timekeeping device that uses the known atomic resonance frequency of an atom to keep time. The first truly accurate atomic clock was built in 1955 at the National Physical Laboratory in the UK and was based on the caesium atom -133 which oscillates at exactly 9,192,631,770 every second.

This oscillation is actually a repetitive signal from the microwave radiation emitted by electrons in an atom when they change energy levels. Much of an atomic clock is designed to create the correct state to cause and augment oscillations.
Although other atoms can be used, the oscillation (9,192,631,770 a second) of the caesium -133 atom is now accepted by the International System of Units as being the definition of one second.

Atomic clocks are generally very large and constitute many highly technical apparatus such as vacuums and require whole teams of scientists to maintain and monitor the clocks. Much of which goes into compensating for  unwanted side-effects such as frequencies of other atoms in the clock and even gravitational dilation (where according to Einstein’s theory clocks at different heights run differently because of the differences in the gravitational field)  This makes atomic clocks highly expensive.

Fortunately many large scale national physical laboratories transmit radio time signals from their atomic clocks which can be used to synchronise standard crystal oscillators too.

Atomic clocks are also the basis of GPS (Global Positioning System) as each satellite contains an atomic clock as accurate time is integral for positioning (a position anywhere is made up of a direction, a velocity and time).
GPS signals can also be used to capture a time signal. This is now the most common way computer networks retain accurate time which is also essential in many communications and applications.

Most computer networks use a NTP server (Network Time Protocol) to synchonise their devices to an atomic time signal received via the GPS network.

A universal timescale, UTC (Coordinated universal Time), has been developed based on the time told by atomic clocks, TAI (International Atomic Time). UTC accounts for the slowing of the Earths rotation by adding leap seconds to TAI so as to prevent the gradual drift of night into day (although that would take 40,000 years or so) and allows the whole world to communicate using the same timescale.