NIST Time and Frequency   UTC   CST/CDT       Note-displayed time is independent of your computer's hardware or system clock. NIST Global Positioning System (GPS) Data Archive - NIST continuously monitors the GPS signals from Boulder, Colorado and compares the frequency standard on each satellite to the NIST frequency standard. Commercially available GPS receivers often provide a 1 pulse per second (pps) timing output, and standard frequencies such as 1, 5, and 10 MHz. Properly designed GPS receivers can provide traceability to the national frequency standard maintained by NIST (read more about using GPS for NIST traceability). Time Standards U.S. Naval Observatory U.S. Naval Observatory USNO Master Clock Animated GIF Clocks   Compute Local Apparent Sidereal Time     Time Comparisons     Eraser Clock2     Note-displayed time drifts with your computer's hardware or system clock. GPS Time Transfer - The U.S. Naval Observatory (USNO) monitors the timing of the GPS to provide a reliable and stable coordinated time reference for the satellite navigation system. The USNO provides two modes of operation to monitor the GPS: the Standard Positioning Service (SPS) and the Precise Positioning Service (PPS). The USNO SPS consists of an STel 502 coarse acquisittime.html ion (C/A) code timing receiver and the processed data are available on the USNO Automated Data Service. International Standard Date and Time Notation The International Standard ISO 8601 specifies numeric standard representations of date and time. This standard helps to avoid confusion in international communication caused by the many different national notations and increases the portability of computer user interfaces. In addition, these formats have several important advantages for computer usage compared to other traditional date and time notations. The time notation described here is already the de-facto standard in almost all countries and the date notation is becoming increasingly popular, especially in computer applications. Precise Time and the Master Clock The U.S. Naval Observatory (USNO) is charged with the responsibility for precise time determination and management of time dissemination . Modern electronic systems, such as electronic navigation or communications systems, depend increasingly on Precise Time and Time Interval (PTTI). Examples would be the ground-based LORAN-C navigation system and the satellite-based Global Positioning System (GPS). These systems are based on the travel time of the electromagnetic signals: an accuracy of 10 nanoseconds (10 one billionths of a second) corresponds to a position accuracy of 10 feet. In fast communications, time synchronization is equally important. All of these official systems are referenced to the USNO master clock. Radio Station WWV - NIST offers several alternatives for accessing NIST time by telephone. To hear the same WWV time announcements by phone that you would hear using a shortwave radio, call (303) 499-7111. This is not a toll-free call, except in the local Boulder/Denver, CO, area. Your call will be automatically cut off after approximately 3 minutes. To hear the similar WWVH time announcements from Hawaii, call (808) 335-4363. NIST also offers an Automated Computer Time Service (ACTS) that is designed to send a digital time code signal in ASCII format directly to your computer by phone. To access this service by modem use 8 data bits, no parity, and 1 stop bit. The phone number is (303) 494-4774; this is not a toll free number.   WWV WWVB Records Systems of time Atomic time , with the unit of duration the Systeme International (SI) second defined as the duration of 9,192,631,770 cycles of radiation corresponding to the transition between two hyperfine levels of the ground state of cesium 133. TAI is the International Atomic Time scale, a statistical timescale based on a large number of atomic clocks. International Earth Rotation Service (IERS)     IERS Bulletin A -- Rapid Service/Prediction of Earth Orientation     IERS Bulletin B ("Final" EOP values)     IERS Bulletin C (Leap Second Announcements)     IERS Bulletin D (Announcements of DUT1) Innovation: GPS and Leap Seconds, Time to Change? GPS World, Nov 1999 (Updated almost daily) UTC GPS TAI TT (TDT) ----+---------+-----------------+------------------------+----- | | | ET 1984.0 |<--34s Leap Seconds------->|<----32.184S fixed----->| | | | | |<--15s-->|<----19s-------->| | | fixed | | | | -----+----- DeltaT = 32.184 s + (TAI-UTC) - (UT1-UTC) ---+----- UT1 (UT) 66.23418 s 34 s -0.05018 s TT (TDT) UTC is variable with respect to UT1 and is kept within ±0.9s with leap seconds. The differences between GPS Time and International Atomic Time (TAI) and Terrestrial Time (TT), also know as Terrestrial Dynaminal Time (TDT), are constant at at the level of some tens of nanoseconds while the difference between GPS Time and UTC changes in increments of seconds, each time a leap second is added to UTC time scale. Technical Articles (Datum) - Datum designs, manufactures and markets a broad range of high-tech time and frequency solutions used for the synchronization of information flow in telecommunications, computer networks, satellite networks and test and measurement applications. time.nist.gov 192.43.244.18 time.apple.com 17.254.0.26 ntp2.usno.navy.mil 192.5.41.209 tick.usno.navy.mil 192.5.41.40 tock.usno.navy.mil 192.5.41.41 ns-2.iastate.edu 129.186.140.200 (tock.iastate.edu) kerberos-2.iastate.edu 129.186.78.152 time.iastate.edu 129.186.1.200 The International Occultation Timing Association (IOTA)   Allan Variance -- Overview by David W. Allan   The Science of Timekeeping (HP application note 1289)   KIWI - Precision Timestamp Utility (DOS)   Tick-Tock Atomic Clock   Astronomical Times       © Copyright 2010 - Samuel J. Wormley   by swormley1@gmail.com