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

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GMT vs. UTC vs. CUT vs. UT

London's tower of Big Ben, the bell 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).

UTC (Coordinated Universal Time)/GMT (Greenwich Mean Time) is now 21.26.35

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).

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).

Select Time Zone ... UTC+13 at 180º E [Mike] NZDT (New Zealand Daylight) UTC+11 at 180º E [Lima] NZT,NZST (New Zealand Std. Time), IDLE (Intl. Date Line East) UTC+11 at 165º E [Kilo] EADT/AEDT (East Aus. Daylight), Noumea, New Caledonia UTC+10 at 150º E [Juliet] EAST/AEST (East Aus. Standard Time), Sydney; Guam; Russia Zone 9 UTC+9   at 135º E [India] JST (Japan Standard Time), ROK (Rep. of Korea), Russia Zone 8 UTC+8   at 120º E [Hotel] CCT (China Coast Time), AWST (Aus. West. Std.) & Russia Zone 7 UTC+7   at 105º E [Golf] West Australian Standard Time, Bangkok, JAVT (Java) UTC+6   at   90º E [Foxtrot] ZP6 Russian Zone 5, Dacca UTC+5.5 IST (India Standard Time) UTC+5   at   75º E [Echo] ZP5 Russian Zone 4, Karachi UTC+4   at   60º E [Delta] ZP4 Russian Zone 3, Baku UTC+3   at   45º E [Charlie] EET (Eastern Europe), MSK Moscow Russia Zone 1, BT (Baghdad Time) UTC+2   at   30º E [Bravo] MET (Middle Europe Time), Cairo UTC+1   at   15º E [Alpha] CET (Central European Time), GB (GMT daylight savings), Paris UTC+0   at     0º E [Zulu] is GMT (Greenwich Mean Time), WET (Western European Time) UTC-1   at   15º W [November] WAT (West Africa Time), AT (Azores) UTC-2   at   30º W [Oscar] GST Greenland, Recife UTC-3   at   45º W [Papa] Atlantic Daylight Time, Rio UTC-3.5 at   45º W [Papa] NST, NFT Newfoundland UTC-4   at   60º W [Quebec] AST (Atlantic Standard Time), Caracas, Amazon UTC-5   at   75º W [Romeo] EST (Eastern Standard Time), ACT (Acre, Brazil) UTC-6   at   90º W [Sierra] CST (Central Standard Time), Chicago UTC-7   at 105º W [Tango] MST (Mountain Standard Time), Denver UTC-7   at 105º W [Tango] Arizona Time (no Daylight Savings) UTC-8   at 120º W [Uniform] PST (Pacific Standard Time) UTC-9   at 135º W [Victor] YST (Yukon Standard Time), Juneau Alaska UTC-10 at 150º W [Whiskey] AHST (Alaska) HST (Hawaii Standard Time) UTC-11 at 165º W [XRay] CAT (Central Alaska), NT (Nome) & Samoa Time UTC-12 at 180º W [Yankee] IDLW (International Dateline West)

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.

After much work, I created a spreadsheet that reconciles the various naming conventions. I make it available to people who help us keep it updated. Email me for it so I can keep you updated.

My file is integrated from several sources:
Time Service Dept., U.S. Naval Observatory, Washington, DC

ftp:// elsie.nci.nih.gov/pub/

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 ftp://elsie.nci.nih.gov/pub/ archived at ftp://munnari.oz.au/pub/oldtz/.

   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:

   #TZ=Asia/Calcutta
   
   #export TZ
   

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

   #zic -v brazil.zones
   

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

   /usr/sbin/timeconfig 
   

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
   

Time Zone (tz) files are rather dated for:
Oracle 5.5

Fabrizio Pollastri's The Time of Internet

A canonical list of locales and language codes in html and Markup Language is maintained by the Unicode Consortium's CLDR (Common Locale Data Repository) Project. They also maintain the Universal Character Encoding codes for all languages.

Spring Forward, Fall Back

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.

Microsoft Windows doesn't provide an auto-adjust option for the time zone covering Jerusalem because Israel bases daylight time on its lunar Jewish calendar 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.

Personal World Clock
Meeting Planner
across 4 time zones!

Time Zone Names, Abbreviations and other info for for major cities in the world.



Michael Kellstrand's MIDP application calculates the time of sunrise and sunset for a zip code or coordinates.

Pearson's Time Zone functions in Excel

Microsoft on Preparing for daylight saving time changes in 2007 warns that "If meetings between March 11, 2007 and April 1, 2007 are scheduled after the operating system patches are applied but before [Microsoft's TimeZone update] tool is run, they will erroneously be moved one hour earlier."

To calculate time between two time zones, W3C Working with Time zones notes that one should obtain the time in two time zones rather than just adding or subtracting msecs from a timestamp.

Time Sychronization Signals

There are several ways to obtain official time sync signals:

• USNO NTP Network Time Servers
• Radio. The U.S. government National Institute of Standards and Technology (NIST) broadcasts time signals over several high frequency (HF) radio frequencies from stations WWV at Fort Collins, Colo. (303) 499-7111 and WWVH in Kauai, Hawaii (808) 335-4363.

  To support high level applications such as network time synchronization and frequency calibrations as well as consumer electronic products like wall clocks, clock radios, and wristwatches, US-NIST radio station WWVB (also in Ft. Collins) and UK's National Physical Laboratory (NPL) transmits MSF 17KW Radio Time Signal (from Anthorn in Cumbria, England at latitude 54° 55' N, and longitude 3° 15' W) Rugby Time short wave signals at 60Khz.

• A dial-up modem can obtain time signals from NIST in Baltimore, Md.
• Trimble's Acutime 2000TM Synchronization Kit receives signals from a GPS satellite.

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.

List of Receiver Manufacturers

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

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.

Display Machine Time and Date

From Programs | Accessories | Command Prompt window on any machine, type these commands:
date
The current date is: Thu 6/5/2025
Enter the new date: (mm-dd-yy)

time
The current time is: 21:26:35.79
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

Client-Server Time

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).

Configuring Windows XP vs WIndows 2003/2000 NIST's time servers.

Microsoft on Synchronizing your computer clock


This alarm clock, by Kisho Kurokawa, reverses the traditional orbit of numbers (among other clock/watch crazy architects)


Pencil and eraser on paper

Computer Network Time Synchronization (Hardcover, CRC March 24, 2006) by David L. Mills This is the "Bible of NTP", describes the internals of the Network Time Protocol and time synchronization.

Sync With a Public Time Server



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 /manualpeerlist:time.nist.gov,0x8 /syncfromflags:MANUAL

The w32tm program has this command set obtained from:

w32tm /?

Windows 2000 has another command syntax.

Designate Type of Windows Time Service

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 ntp.org.

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.

Designate a Public/External NTP Time Service

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:

NET TIME \\%COMPUTERNAME% /SETSNTP:ntp-ucla.usno.navy.mil

NET TIME \\%COMPUTERNAME% /setsntp:ntp2.usno.navy.mil

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 time.windows.com domain to an internal server.

To confirm the SNTP value:
NET TIME /querysntp

Setup Domain Time Server

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.

Adjust Kerberos default time skew for network speed and security

Unlike NT, Windows 2000 domain controllers need to have their time closely synchronized for Kerberos , 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.

Flag a Domain Controller as a Reliable Time Source

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.

Synchronize Client Time

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

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 freeware such as Dimension 4, Webtime, or other Winsock32 SNTP clients. Atomtime.

Microsoft Articles:

• How to Configure an Authoritative Time Server in Windows 2000
• Basic Operation of the Windows Time Service
• Windows 2000 Utilities: Taming the Time Service
• Registry Entries for the W32 Time Service
• The Windows 2000 Time Service

Universal Coordinated Time

UTC is obtained from a Stratum 2 NTP time server which in turn sets itself to a trusted source (stratum 1 server) such as the US Naval Observatory's Atomic Clock time server (tick.usno.navy.mil) in Washington, DC, which maintains the official US Standard Time. NIST, the National Institute of Standards and Technology, located in Boulder, Colorado, provides the Automated Computer Time Service (ACTS), which can set a computer clock with an uncertainty of less than 10 milliseconds. Many sites at various time zones throughout the world offer time synchronization services.

Time Sync Software

Comparison of the $495 Greyware utility vs. Microsoft's W32Time and others:

NetTime from Sourceforge

$19.95 Altrixsoft Chronograph software synchronizes your local PC time using the Internet time service.

Clock display programs such as the $9.95 WorldTimer (30 day nagware) use SNTP to set their time.

$13 JCOM Clock Synchronizer

Time Related Websites

• www.ubr.com/clocks
  Everything you ever wanted to know about clock history, clock making, sundials, time standards, and other topics. Links to websites for time-related newsgroups, museums, books, Ezines, and more.

• www.calendarzone.com
  A feast of celestial, cultural, historical, geographical, religious, and interactive calendar-related information. Y2K and millennium discussions plus calendar software and reference links.

• www.earthcalendar.net
  A second calendar marvel. Lunar phase information, holidays sorted by religion and country worldwide. Links to related sites.

• tycho.usno.navy.mil/time.html
  Time service of the United States Naval Observatory, Washington, D.C. Click for universal time, US time zones, where to see the first sunrise of the third millennium, and other information. Links to astronomical information.

• www.studyoftime.org
  Website for the International Society for the Study of Time. Information on the history and goals of this interdisciplinary society, publications, and conferences.

• www.lindahall.org/ pubserv/ hos/ Y1K/ index
  Online exhibition of science and technology in the year 1000 (Y1K). Also examines developments in medicine, astronomy, algebra, alchemy, and horticulture that grew out of the fertile intellectual climate surrounding the first millennium. Service of the Linda Hall Library of Science, Engineering, and Technology, Kansas City, Missouri.

• www.wnet.org/ archive/ tenement/ floorboards.html
  Website for the Lower East Side Tenement Museum in New York. Peel back layers of wallpaper and lift floorboards to discover objects found in this tenement turned archaeological site turned museum. Boarded up from 1935-1987, the building is a living time capsule. Dollhouse dioramas, photographs, history, and more.

• aic.stanford.edu
  The American Institute for Conservation of Historic and Artistic Works. It provides a wealth of information regarding the preservation and conservation of objects in time. Includes links to related sites.

• www.cam.net.uk/ home/ pb/ index.html
  A brief history of 24-hour time, otherwise known as military, railroad, or navigational time. Links to sources for 24-hour clocks.

NTP on Linux

NTP.org

Accurate Global Time Synchronization - Managing Accurate Date and Time on Linux

List of public Stratum 2 NTP servers

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.

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

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.

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.

Humanclock.com 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 the source code for this Java applet which displays your local time

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

A clock for aliens in outer space

The gold plaque on the Pioneer 10 spacecraft (designed by Carl Sagan and Frank Drake), now traveling at 27,380 mph through deep space toward the star Aldebara in the eye of the Taurus constallation, uses lines to represent earth's distance from 14 known pulsars (rotating neutron stars that send out on/off pulses of light like a lighthouse beacon). Because Pulsars are known to be slowing down, ticks on the bars can indicate in binary form the interval between pulses (the periods of pulsars) on the date Pioneer was launched in 1972.

The fastest pulsar, PSR 1937+21, spins on its axis 640 times per second (faster than a kitchen blender!). The strongest pulsar, PSR 0329+54, has a pulse period of 0.715 seconds.

A light year is 9,460,550,000,000,000 metres, based on
the speed of light at 299,792,458 metres per second ( m/s ).

A parsec is approx. 3.25 light years

Latham's calendar for the planet Mars is composed of equal 167-day quarters. Each quarter contains 3 months, configured as (56 + 56 + 55) days. The Martian clock day is defined as 25 hours of 53 minutes of 67 standard Terran seconds. A simple leap year rule makes common years of years 2, 4, 6 and 8 of each 10 years, and makes an exception in the tenth year every 2000 years, yielding a calculated accuracy of one day in thirty-seven thousand years. The seven-day week is retained.