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(E) Atomic Clock
By Nenad N. Bach | Published  01/20/2002 | Miscellaneous | Unrated
(E) Atomic Clock Your Computer Clock Via the Internet NIST Internet Time Service (ITS)Software and InstructionsPublic Domain NIST Software  • Windows 95 and later (32 or 64-bit)  • Windows 3.1 (16-bit)  • FTP Site (all files & source code)  Instructions  • Macintosh Computers (PDF file)  • Windows Computers (PDF file)  • List of Software PublishersThe NIST Internet Time Service allows users to synchronize computer clocks via the Internet. The time information provided by the service is directly traceable to UTC(NIST). The service responds to time requests from any Internet client in several formats including the DAYTIME, TIME, and NTP protocols.The NIST Internet Time Service uses multiple stratum-1 time servers. Here are the server names, locations, and IP addresses.Time Code FormatsInternet time code protocols are defined by a series of documents called Request for Comments, or RFCs. These documents are available on-line from several sites on the Internet. The protocols supported by the NIST Internet Time Service are:Daytime Protocol (RFC-867)This protocol is widely used by small computers running MS-DOS and similar operating systems. The server listens on port 13, and responds to requests in either tcp/ip or udp/ip formats. The standard does not specify an exact format for the Daytime Protocol, but requires that the time is sent using standard ASCII characters. NIST chose a time code format similar to the one used by its dial-up Automated Computer Time Service (ACTS), as shown below:JJJJJ YR-MO-DA HH:MM:SS TT L H msADV UTC(NIST) OTMwhere:* JJJJJ is the Modified Julian Date (MJD). The MJD is the last five digits of the Julian Date, which is simply a count of the number of days since January 1, 4713 B.C. To get the Julian Date, add 2.4 million to the MJD.* YR-MO-DA is the date. It shows the last two digits of the year, the month, and the current day of month.* HH:MM:SS is the time in hours, minutes, and seconds. The time is always sent as Coordinated Universal Time (UTC). An offset needs to be applied to UTC to obtain local time. For example, Mountain Time in the U. S. is 7 hours behind UTC during Standard Time, and 6 hours behind UTC during Daylight Saving Time.* TT is a two digit code (00 to 99) that indicates whether the United States is on Standard Time (ST) or Daylight Saving Time (DST). It also indicates when ST or DST is approaching. This code is set to 00 when ST is in effect, or to 50 when DST is in effect. During the month in which the time change actually occurs, this number will decrement every day until the change occurs. For example, during the month of October, the U.S. changes from DST to ST. On October 1, the number will change from 50 to the actual number of days until the time change. It will decrement by 1 every day until the change occurs at 2 a.m. local time when the value is 1. Likewise, the spring change is at 2 a.m. local time when the value reaches 51.* L is a one-digit code that indicates whether a leap second will be added or subtracted at midnight on the last day of the current month. If the code is 0, no leap second will occur this month. If the code is 1, a positive leap second will be added at the end of the month. This means that the last minute of the month will contain 61 seconds instead of 60. If the code is 2, a second will be deleted on the last day of the month. Leap seconds occur at a rate of about one per year. They are used to correct for irregularity in the earth's rotation. The correction is made just before midnight UTC (not local time).* H is a health digit that indicates the health of the server. If H=0, the server is healthly. If H=1, then the server is operating properly but its time may be in error by up to 5 seconds. This state should change to fully healthy within 10 minutes. If H=2, then the server is operating properly but its time is known to be wrong by more than 5 seconds. If H=4, then a hardware or software failure has occurred and the amount of the time error is unknown.* msADV displays the number of milliseconds that NIST advances the time code to partially compensate for network delays. The advance is currently set to 50.0 milliseconds.* The label UTC(NIST) is contained in every time code. It indicates that you are receiving Coordinated Universal Time (UTC) from the National Institute of Standards and Technology (NIST).* OTM (on-time marker) is an asterisk (*). The time values sent by the time code refer to the arrival time of the OTM. In other words, if the time code says it is 12:45:45, this means it is 12:45:45 when the OTM arrives. Time Protocol (RFC-868)This simple protocol returns a 32-bit unformatted binary number that represents the time in UTC seconds since January 1, 1900. The server listens for Time Protocol requests on port 37, and responds in either tcp/ip or udp/ip formats. Conversion to local time (if necessary) is the responsibility of the client program. The 32-bit binary format can represent times over a span of about 136 years with a resolution of 1 second. There is no provision for increasing the resolution or increasing the range of years.The strength of the time protocol is its simplicity. Since many computers keep time internally as the number of seconds since January 1, 1970 (or another date), converting the received time to the necessary format is often a simple matter of binary arithmetic. However, the format does not allow any additional information to be transmitted, such as advance notification of leap seconds or daylight savings time, or information about the health of the server.Network Time Protocol (RFC-1305)The Network Time Protocol (NTP) is the most complex and sophisticated of the time protocols, and the one that provides the best performance. NTP is typically used by large computers and workstations, since NTP software is often bundled with the operating system. The client software runs continuously as a background task that periodically gets updates from the server. The client software can be configured to query several servers, and to average the results. By querying multiple servers, it can ignore responses from servers that appear to be sending the wrong time.The NIST servers listen for a NTP request on port 123, and respond by sending a udp/ip data packet in the NTP format. The data packet includes a 64-bit timestamp containing the time in UTC seconds since January 1, 1900 with a resolution of 200 picoseconds. Since the client software runs continuously, it can keep the client's clock within a few milliseconds of UTC(NIST).For questions or more information about the NIST Internet Time Service, contact Judah Levine: jlevine@boulder.nist.govdistributed by CROWN - - CroWorldNet@aol.comNotice: This e-mail and the attachments are confidential information.If you are not the intended recipient of this e-mail, you are hereby notified that any dissemination, distribution or copying of this e-mail and the attachments is strictly prohibited and violators will be held to the fullest possible extent of any applicable laws governing electronic Privacy. If you have received this e-mail in error please immediately notify the sender by telephone or e-mail, and permanently delete this e-mail and any attachments.
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