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Reload this page Clocks and Time Keeping

Here are notes on clocks and time-keeping. This is a companion to pages on Date Programming and Scheduling.

Stopwatch 1: ms
Stopwatch 2: s
Countdown from:

Stopwatch timer


Topics this page:

  • Time Zones
  • Daylight Savings
  • Time Sync.
  • Time Sync. NT
  • Linux
  • Your Clocks
  • Just a Second
  • Space Clock
  • Your comments???

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    Set this at top of window. GMT vs. UTC vs. CUT vs. UT

      wav sound London's tower of Big Ben, the bell

      The Big Ben bell clock tower Parliament Building

      In the US Navy (unofficially), a "Pollywog" is a seaman who has not traveled across the Equator. A "Shellback" is someone who has crossed the Equator. An "Emerald Shellback" is someone who has crossed the Equator at the Prime Meridian. A "Golden Shellback" is someone who has crossed the Prime Meridian at the 180th Meridian (International Date Line). A "Golden Dragon" is someone who has Crossed the 180th Meridian (International Date Line).

      Javascript feature UTC (Coordinated Universal Time)/GMT (Greenwich Mean Time) is now

      The "legal" time in both the UK (United Kingdom) and the USA is GMT.

      "GMT" is an abbreviation for "Greenwich Mean Time", named for the Greenwich Observatory near Greenwich Observatory near London, England (in Great Britain / the United Kingdom) where a system was first developed around 1850 for tracking time based on the rotation of the Earth.

      Today, time signals in the UK and most other parts of the world are based on "UTC time", which is based on vibrations of crystals.

      Practically, the terms GMT and UTC, CUT, and UT are used synonomously because the difference between them should never exceed 0.9 seconds due to leap seconds and leap years — 55 ms (1.0/18.2 Hz).

      According to a NIST webpage:

        In 1970 the Coordinated Universal Time system was devised by an international advisory group of technical experts within the International Telecommunication Union (ITU). The ITU felt it was best to designate a single abbreviation for use in all languages in order to minimize confusion. Since unanimous agreement could not be achieved on using either the English word order, CUT, or the French word order, TUC, the acronym UTC was chosen as a compromise.

      The prefered "best practice" method of running production servers is with the timezone set to UTC (GMT).

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    Set this at top of window. Time Zones

      The one-hour difference between most Time zones aggregate the time that one moves away from any given point on Earth, such as the Prime Meridian (at GMT/UTC/CUT/UT).

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      At the Equator, each time zone hour spans about 1,041 miles (approximately 25,000 miles / 24).
      However, because the Earth is a globe, the circular lines of longitude gets smaller as one moves north or south away from the equator and toward one of the poles. At 45 degrees N or S of the equator, the distance between one-hour time zones shrinks to about 739 miles.

      The -13 is for Daylight Savings from the -12 time zone. This is why -13 is still called "Mike" in the US Military alphabet.

      Timezones on UNIX machines were written using a file containing a set of time code names created based on the political landscape of the early 1970s (before Burma became Myanmar in 1989 and Zaire became Congo on May 17, 1997 after Mubutu). Some "tz" codes differ from names assigned by other political entities.

      Microsoft's graphical user interface does away with time codes entirely.

      To perform timezone conversions, use standard timezone libraries
      ICU4C for C/C++ from IBM and ICU or
      Joda time.

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      Set this at top of window. Time Zones on Solaris

        To set how time is displayed on a Solaris in say, IST (India Standard Time):

      1. As root, execute the Zone Information Compiler with this command:

        #zic /usr/share/lib/zoneinfo/asia

        This creates a file named "asia" from an complexly formatted ASCII source file that is not distributed. However, Linux operating systems (such as RedHat) use the same files and do distribute them. So:

      2. Copy the /etc/TIMEZONE file from another distribution or from archived at

        Some also refer to this as the "Olson timezone database"

      3. Edit the /etc/TIMEZONE file to set the TZ variable to "Asia/Calcutta".
      4. As root, execute:

        #export TZ
      5. Another example: To change the date which daylight savings time switches,

        #zic -v brazil.zones

      Set this at top of window. Time Zones on Linux

        To Configure local time on Linux boxes:

      1. Set the machine CMOS
      2. backup /etc/localtime file:

        # mv /etc/localtime /etc/localtime.bak

      3. the time can be set interactively using the command


      4. to ensure that the machine starts to the same time zone every time, add in the server startup script:

        # cp /etc/localtime /etc/localtime.bak
        # cp /usr/share/zoneinfo/America/mycity /etc/localtime

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    Set this at top of window. Spring Forward, Fall Back Reminder

      Daylight Saving Time shifts more daylight into longer Summer evenings. Owners of evening venues (such as theaters) don't like it.

      In low latitudes around the equator, DST makes little difference, so DST is NOT observed in Hawaii, American Samoa, Guam, Puerto Rico, the Virgin Islands. DST is also not observed in the state of Sonora in Mexico.

      DST is also not observed in the sunny state of Arizona (except within the Navajo Indian Reservation, which does observe since it spans 3 states). So Arizona farmers share their clock with California during the Summer and with Mountain time the rest of the year.

      "Indiana Time" -- which was in the Eastern Time Zone but does not adjust clocks for DST -- was abolished with the time change on April 2, 2006. Previously, 77 counties in Indiana were on "Indiana Time". But one still needs a map to tell time in Indiana, since the US Dept. of Transportation allows 74 counties to be on Eastern time and 18 counties on Central time.

      In the U.S., since 1987, each time zone switches generally at 2 a.m. local time on the first Sunday of April and the last Sunday of October.

      In 2007 and onward, DST begins on the 2nd Sunday in March and end the first Sunday of November. "Extended DST" starts three weeks earlier and ends a week later than before due to the Energy Act of 2005 George W. Bush signed Aug. 2005.

      Three new time zones have been created for Mexico because Mexico decided not to follow the new DST rules of U.S. and Canada. Most of Mexico used Central Standard Time. Baja California Norte followed Pacific Standard Time with its northern neighbor. Mountain Standard Time was observed by Baja California Sur and the central states of Chihuahua, Nayarit, Sonora, Sinaloa.

      Caution! Microsoft Windows doesn't provide an auto-adjust option for the time zone covering Jerusalem because Israel bases daylight time on its lunar Jewish calendaranother page on this site Also, Palestinians change their clocks at different times (as an assertion of independence).

      The European Union changes to "Summer Time" at 1 am UTC(GMT) the last Sunday in March (earlier than in the U.S.) and the last Sunday in October.

      Brazil does not have such an annual designation, but announced at possibly a different day every year.

      The Faroes Islands, the offset changes twice a year.


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    Set this at top of window. Sun Dials

    Set this at top of window. Time Sychronization Signals


      Some clocks never need resetting because they automatically adjust for daylight savings time and keeps itself accurate ten billionths of a second by synchronizing itself to radio signals.

      A website external to this site List of Receiver Manufacturers

      tool The free metronome from GCH Guitar Academy ticks from 40 to 160 bpm with signatures in 3/4, 4/4, and 6/8 time.

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    Set this at top of window. Windows Time synchronization for a "Good Time"

      It's important that the time on machines are closely synchronized, especially when time-stamped logs from several machines on a server farm will be analyzed together.

      The real-time clock (RTC) hardware on machines can be off by an hour or more after a week or two without correction. Machine times may also drift at different rates. So under typical conditions, a delta of up to 20 milliseconds (0.020 seconds) is considered a reasonably exact match. The clock on a Windows 2000 system ticks approximately once every 10 milliseconds.

      If a machine's time is too "loosely" synchronized with a server, expect this message:

        The system cannot log you on due to the following error:
        There is a time difference between the Client and Server.

    1. Display Machine Time and Date
    2. From Programs | Accessories | Command Prompt window on any machine, type these commands:

        The current date is:
        Enter the new date: (mm-dd-yy)

        The current time is:
        Enter the new time:

      This command changes both the BIOS clock (which a battery on your motherboard keeps ticking when your computer is powered off) and the operating system's clock on your Task Bar.

      Get the TClockE utility to put a date (and other info) on your Task Bar

    3. Client-Server Time
    4. Microsoft Windows 2000 has a built-in Net Time command which can be called by custom login batch scripts.

      To get the time of the domain controller on a domain named "domain1", from Start | Programs | Accessories | Command Prompt window, type:

        net time /domain:domain1
        Current time at \\computer1 is 6/14/200x 3:24 AM
        The command completed successfully.

      To get the time of a computer by the NETBIOS name obtained from environment variable "computer1", type:

        net time \\%COMPUTERNAME%
        Current time at \\computer1 is 6/14/200x 3:24 AM
        The command completed successfully.

      If you get get a "Bind Failed" message, stop W32Time and try again. (see next item).


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    1. Sync With a Public Time Server
    2. To sync up with an authoritative time server on the internet, there are two alternatives:

      On Windows XP, double click on the task bar. Click the ":Internet Time": tab if it's there and click ":Update Now":

      An error can occur if your firewall blocks the port that is used.

      To manaully sync up with an authoritative time server on the internet:

      w32tm /config /,0x8 /syncfromflags:MANUAL

      The w32tm program has this command set screen captured obtained from:

        w32tm /?

      Windows 2000 has another command syntax.

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    1. Designate Type of Windows Time Service
    2. To keep accurate time, Windows 2000 Server by default invokes upon startup the Windows Time service ("W32Time.dll").

      On Windows 2000 Professional, stop W32time in Start | Settings | Control Panel | Administrative Tools | Services | Windows Time. Alternately, to manually start it from a Command Prompt, Run:

        net start w32time

      Since W32Time is started by default, the expected response to the command above is:

        The requested service has already been started.
        More help is available by typing NET HELPMSG 2182.

      W32Time is controlled by values set in registry subkey HKEY LOCAL MACHINE\ SYSTEM\ CurrentControlSet\ Services\ W32Time\ Parameters.

      Windows 2000-based servers have a default "Type" REG_SZ value of "Nt5DS" for "Domain hierarchy based synchronization", which means that when a Windows 2000 workstation starts up on a network (not local), the Net Logon service looks for a domain controller that can authenticate and synchronize time with a domain controller in its own domain.

      This can also be done manually:

        net time /domain /set
        Current time at \\computer1 is 6/14/200x 3:24 AM
        The command completed successfully.

      The syntax of the net time program is not case sensitive:

        net time /?

        NET TIME
        [\\computername | /DOMAIN[:domainname] 
        	| /RTSDOMAIN[:domainname]] [/SET]
                 [\\computername] /QUERYSNTP
                 [\\computername] /SETSNTP[:ntp server list]

      In turn, domain controllers in child domains synchronize their time with the Forest Root (Parent) Domain controller.

      The "Type" can be changed to "NoSync" for no synchronization if you are using a third-party time utility. NET TIME has limitations versus 3rd party utilities such as Greyware's $495 Domain Time server control panel applet, which runs as a background service with safeguards against flooding attacks.

      The "Type" can be changed to "NTP" for time synchronization with an external time service using the "industry-standard" March 1995 SNTP (Simple Network Time Protocol) [RFC 1769] (which obsoletes RFC 1361) and is obsoleted by October 1996 SNTP version 4 [RFC 2030], all adaptations of the full NTP version 3 [RFC 1305] maintained by

      Reminder NTP aims for accuracy of +/- 10 ms. The SNTP protocol aims to keep all clocks in the machines of an enterprise within 20 seconds of one another, and all clocks in a site within two seconds of one another. Is this enough for you? Maybe not.

      NTP Time Server on Windows from Meinberg, German makers of NTP cards and devices, provides on Windows machines the same NTP service that runs on Unix servers. Its Monitor Windows GUI displays NTP related application log entries and time skew on a graph.

    3. Designate a Public/External NTP Time Service
    4. A machine would use external synchronization if it's a stand-alone machine or if it's a domain's time service, which is by default the Forest Root (Parent) Domain controller (PDC emulator). However, in a large forest, designate another domain controller to be the Domain Time Server for the entire forest.

      Server clocks are synchronized to the “UTC”, which English speakers call the Coordinated Universal Time. BTW, the letters of this acronym is purposely jumbled from its acronymn due to a compromise between the English "CUT" and French acronym "TUC" for the same concept. This accord was reached in 1970 by the International Telecommunication Union (ITU) of technical experts from around the world who sought a replacement for "Greenwich Mean Time" near London, England. UTC standard time is independent of time zones.

      Note that Windows 2000 only recognizes the first DNS or IP address in the "ntp server list". To set the "NtpServer" REG_SZ value in the W32Time\Parameters registry subkey, run one of these:



        The expected response is The command completed successfully.

        If you get an "Access Denied" message, make sure you have Administrator rights to start and stop services.

        If you get message "Could not locate a time server" make sure that your firewall allows UDP port 123, which Windows external time services use by default.

        Some companies reroute the Windows default domain to an internal server.

      To confirm the SNTP value:

        NET TIME /querysntp

    5. Setup Domain Time Server
    6. Designate a computer to be the time server computer by using RegEdit to navigate to the HKEY LOCAL MACHINE \SYSTEM \CurrentControlSet \Services \W32Time \Parameters subkey and changing the “LocalNTP” REG_DWORD value from 0 to 1.

      To verify that the time service is synchronizing time, use a tool installed by default in the WINNT/System32 folder:

        w32tm -once -test -v

        This verbosely lists the messages issued in a time synchronization request.

    7. Adjust Kerberos default time skew for network speed and security
    8. Unlike NT, Windows 2000 domain controllers need to have their time closely synchronized for Kerberos another page on this site, which exchanges time-stamped authenticator identification tokens. This is also true for OpenSSL and all encryption methods which are based on time.

      If the server is on a slow network, you may receive this message:

        KRB_AP_ERR_SKEW 0x25 "Clock skew too great"

      Clock Skew is the range of time allowed for a server to accept Keberos authenticators from a client. The default is 5 minutes. Longer time skews allow time for the Net Logon service to perform authentication over slow networks. Even though logon passwords travel over the secure encrypted Net Logon Secure Channel, a discreet communication channel, you may shorten time skews for better protection from replay attacks where "hijackers" intercept communications between a client and a server and replay them to gain access to network resources.

      To change the Kerberos time skew on the server,

      1. From the Start menu, point to Settings, click Control Panel, double-click Administrative Tools, and then double-click Domain Security Policy.
      2. Expand Security Settings, Account Policies, and Kerberos Policy.
      3. Right-click Maximum tolerance for computer clock synchronization.
      4. Click Security.
      5. In the Security Policy dialog box, change the maximum tolerance variable.

      W32Time uses the client's secure account password to generate a signature on SNTP packets that are sent across the network. These contain a signed 64-bit hash of the time information.

    9. Flag a Domain Controller as a Reliable Time Source
    10. Once a domain controller is known to keep accurate time, use RegEdit to mark the computer as reliable by navigating to HKEY LOCAL MACHINE \SYSTEM \CurrentControlSet \Services \W32Time \Parameters subkey and changing the "ReliableTimeSource" REG_DWORD to value "1".

      The Net Logon service should now announce that the computer is a reliable time source when it logs on to the network.

    11. Synchronize Client Time
    12. To manually force re-synchronization from a Command Prompt: use W32Time, which has an evolving set of command syntax:

        Windows XP machines have this command set screen captured

          w32tm /resync /rediscover

        On an NT/2000 client:

          w32tm -s computer1
          RPC to server computer1 returned 0x0

      The program issues Sending resync command to local computer...

      A zero return value means that the command was sent successfully -- not necessarily completed successfully. Unlike NTP, SNTP uses connectionless UDP, which doesn't guarantee delivery.

      By default Windows 2000 machines perform a synchronization once every 45 minutes until successful three times, then once every eight hours (three times per day) This "SpecialSkew" specification can be changed in the registry.

      This keeps "loose synchronization" among all clocks in a network.

      Differences of less than 55 ms do not trigger a time update.

      No PDC?

      PDC's which do not have an Active Directory server can use tool freeware such as Dimension 4, Webtime, or other Winsock32 SNTP clients. Atomtime.

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    Set this at top of window. NTP on Linux


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    Set this at top of window. Just a Second

      In 1900, a second was defined as 1/86400th of a mean solar day (one Earth rotation).
      But there are actually 86164.10 seconds in a sidereal day — the length of time (synodic month) which passes between a given “fixed” star in the sky crossing a given projected meridian. Therefore:

      • 31,536,000 seconds = 1 calendar year
      • 31,558,150 seconds = 1 sidereal year
      • 31,556,930 seconds = 1 tropical year

      Ephemeris Time (ET), used in the Astronomical Almanac from 1960-1983, was replaced in the 1984 Astronomical Almanac which implements the IAU 1976 System of Astronomical Constants

      • Dynamical time for barycentric phenomena (TDB) used as a time scale of ephemerides, referred to as the barycenter — the center of mass (a common axis of rotation) around which the solar system revolves.

      • Terrestrial Dynamical Time (TDT) for geocentric phenomena — which is independent of the variable rotation of the Earth — is used as the time scale of ephemerides (daily table showing planetary positions) for observations from Earth's surface.

      TDT differs from "TAI" an abbreviation due to a compromise between French and English for "International Atomic Time") which measures the International System SI second as the time required for 9,192,631,770 vibrations (transition cycles) of a Cesium (Cs-133) atom in a Cs clock.

      Their offset — always less than 2 milliseconds — is due to variations in the gravitational potential around the Earth's orbit combined with velocity terms.


    The minimalist "Reveal" watch design by Daniel Will Harris (above and below) is the coolest watch I've ever seen.

    Order the matching screen saver as well!

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      Set this at top of window. Split Seconds

      Fraction of a second Metric name Abbr.
      whole 1.0 second s
      hundredth 0.01 centisecond cs
      thousandth 0.001 millisecond ms
      millionth 0.000 001 microsecond µs
      billionth 0.000 000 001 nanosecond ns
      trillionth 0.000 000 000 001 picosecond ps
        0.000 000 000 000 001 femtosecond fs
        0.000 000 000 000 000 001 attosecond as
        0.000 000 000 000 000 000 001 zeptosecond zs
        0.000 000 000 000 000 000 000 001 yoctosecond ys

      Reminder A millisecond is one thousandth of a second, NOT one millionth. This is a common confusion.

      The NIST clock in Boulder, Colorado (at 5400 feet altitude) ticks 5 microseconds faster than an identical clock at Greenwich, England (at 80 feet altitude) because clocks are faster by 10-16 per meter elevation.


      It takes ...

      0.001 Seconds for a high speed camera shutter to snap.

      0.025 Seconds to blink a human eye.

      0.02 Seconds for an average hummingbird wingbeat.

      A nanosecond — one-billionth of a second — for an electric current to travel the length of a human forearm (the ancient cubit). This explains why cables for computer system buses operating at 1GB/second transfer rates must be about 1 foot in length.

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    Set this at top of window. Clock Locations

      Walk around with this checklist. If you find that you've forgotten one, add to this list for next time.
      • __ Wrist Watches
      • __ Beepers
      • __ Computer (Windows 95 will automatically adjust for Daylight Savings)
      • __ Personal Digital Assistants (Palm Pilot)
      • __ Bedroom alarm clocks
      • __ Travel Alarm Clocks
      • __ Wall Clocks
      • __ TV
      • __ VCR
      • __ Boom Box
      • __ Kitchen Coffee brewer
      • __ Kitchen Oven
      • __ Microwave Oven
      • Car

      Most cell phones don't need to be set because the carrier signals they receive contain a time sync.

      webpage article displays a different photo every minute of the day as a clock. Brilliant art.
      View the clock, which auto updates with a picture from various parts of the world.

      This analog clock makes use of the jQuery implementation of the CSS2 rotate images command. Hands on the clock will move only when viewed using Google Chrome, Safari, IE8, or a browser supporting HTML5 which implements CSS2.


    download Download the source code for this Java applet which displays your local time

    Current Phase of the Moon calculator by Ed Stephan

    Alarm Clocks
    Cuckoo Clocks
    Desk Clocks
    Grandfather Clocks
    Kitchen Clocks
    Mantel/Shelf Clocks
    Novelty Clocks
    Pendulum Clocks
    Wall Clocks

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    Set this at top of window. A clock for aliens in outer space

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