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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:
The prefered "best practice" method of running production servers is with the timezone set to UTC (GMT).
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).
At the Equator, each time zone hour spans about 1,041 miles (approximately 25,000 miles / 24).
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.
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:
To set how time is displayed on a Solaris in say, IST (India Standard Time):
To Configure local time on Linux boxes:
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.
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.
across 4 time zones!
Michael Kellstrand's MIDP application calculates the time of sunrise and sunset for a zip code or coordinates.
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."
The time indicated on a sun dial is called Apparent Solar Time, or "true local time".
There are several ways to obtain official time sync 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.
The free metronome from GCH Guitar Academy ticks from 40 to 160 bpm with signatures in 3/4, 4/4, and 6/8 time.
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:
There is a time difference between the Client and Server.
The current date is:
Enter the new date: (mm-dd-yy)
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.
To get the time of the domain controller on a domain named "domain1", from Start | Programs | Accessories | Command Prompt window, type:
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:
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).
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.
Universal Coordinated TimeUTC 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 SoftwareComparison of the $495 Greyware utility vs. Microsoft's W32Time and others:
$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.
Time Related Websites
In 1900, a second was defined as 1/86400th of a mean solar day (one Earth rotation).
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
Their offset — always less than 2 milliseconds — is due to variations in the gravitational potential around the Earth's orbit combined with velocity terms.
Order the matching screen saver as well!
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.
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.
Most cell phones don't need to be set because the carrier signals they receive contain a time sync.
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
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
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.
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