A time signal is a visible, audible, mechanical, or electronic signal used as a reference to determine the time of day. Church bells or voices announcing hours of prayer gave way to automatically operated chimes on public clocks; however, audible signals (even signal guns) have limited range. Busy ports used a visual signal, the dropping of a ball, to allow mariners to check the chronometers used for navigation. The advent of electrical telegraphs allowed widespread and precise distribution of time signals from central observatories. Railways were among the first customers for time signals, which allowed synchronization of their operations over wide geographic areas. Special purpose radio time signal stations transmit a signal that allows automatic synchronization of clocks, and commercial broadcasters still include time signals in their programming. Today, GPS navigation radio signals are used to precisely distribute time signals over much of the world.
Audible and visible time signals 
One sort of public time signal is a striking clock. These clocks are only as good as the clockwork that activates them, but they have improved substantially since the first clocks from the 14th century. Until modern times, a public clock such as Big Ben was the only time standard the general public needed.
When more accurate time signals were required in navigation, a number of traditional audible or visible time signals were established so navigators could check their marine chronometers. These public time signals were formerly established in many seaport cities.
In Vancouver, British Columbia, a "9 O'Clock Gun" is still shot every night at 9 pm. (This gun was brought to Stanley Park in 1894 by the Department of Fisheries originally to warn fishermen of the 6:00 pm Sunday closing of fishing.) The 9:00 pm firing was later established as a time signal for the general population. Until a time gun was installed, the nearby Brockton Point lighthouse keeper detonated a stick of dynamite. A similar "Noon Gun" is still shot every noon at Cape Town, South Africa, as well as at the Citadelle of Quebec. A cannon was shot at one o'clock every weekday at Liverpool, England, at the Castle in Edinburgh, Scotland, and also at Perth in Australia to establish the time. The Edinburgh "One O'Clock Gun" is still in operation. A cannon located at the top of Santa Lucia Hill, in Santiago, Chile, is shot every noon. (In places where a cannon is used for a time signal, locals often joke that they can spot tourists because they jump in surprise while locals check their watches.)
In many Midwestern US cities where tornadoes are a common hazard, the emergency sirens are tested regularly at a specified time (say, noon each Saturday); while not primarily intended to mark the time, locals often check their watches when they hear this signal. In many non-seafaring communities, loud factory whistles served as public time signals before radio made them obsolete. In 1861 and 1862, the Edinburgh Post Office Directory published time gun maps relating the number of seconds required for the report of the time gun to reach various locations in the city.
Because light travels much faster than sound, visible signals enabled greater precision than audible ones, although audible signals could operate better under conditions of reduced visibility. The first time ball was erected at Portsmouth, England in 1829 by its inventor Robert Wauchope. One was installed in 1833 on the roof of the Royal Observatory in Greenwich, London, and the time ball has dropped at 1pm every day since then. The first American time ball went into service in 1845. In New York City, the ceremony of Times Square Ball drop at New Year's Eve in Times Square is a vestige of a visual time signal.
Electrical time signals 
Sandford Fleming proposed a single 24-hour clock for the entire world. At a meeting of the Royal Canadian Institute on 8 February 1879 he linked it to the anti-meridian of Greenwich (now 180°). He suggested that standard time zones could be used locally, but they were subordinate to his single world time.
Standard time came into existence in the United States on 18 November 1883. Earlier, on 11 October 1883, the General Time Convention, forerunner to the American Railway Association, approved a plan that divided the United States into several time zones. On that November day, the United States Naval Observatory telegraphed a signal that coordinated noon at Eastern standard time with 11 am Central, 10 am Mountain, and 9 am Pacific standard time.
A March 1905 issue of The Technical World describes the role of the United States Naval Observatory as a source of time signals:
- One of the most important functions of the Naval Observatory is found in the daily distribution of the correct time to every portion of the United States. This is effected by means of telegraphic signals, which are sent out from Washington at noon daily, except Sundays. The original object of this time service was to furnish mariners in the seaboard cities with the means of regulating their chronometers; but, like many another governmental activity, its scope has gradually broadened until it has become of general usefulness. The electrical impulse which goes forth from the Observatory at noon each day, now sets or regulates automatically more than 70,000 clocks located in all parts of the United States, and also serves, in each of the larger cities of the country, to release a time-ball located on some lofty building of central location. The dropping of the time-ball – accompanied, at some points, with the simultaneous firing of a cannon – is the signal for the regulation by hand of hundreds of other clocks and watches in the vicinity.
Radio time sources 
Dedicated time signal broadcasts 
The telegraphic distribution of time signals was made obsolete by the use of AM, FM, shortwave radio, Internet Network Time Protocol servers as well as atomic clocks in satellite navigation systems. Since 1905 time signals have been transmitted by radio. There are dedicated radio time signal stations around the world.
Time stations operating in the longwave radio band have highly predictable radio propagation characteristics, which gives low uncertainty in the received time signals. Stations operating in the shortwave band can cover wider areas with relatively low-power transmitters, but the varying distance that the signal travels increases the uncertainty of the time signal on a scale of milliseconds.
Radio time signal stations broadcast the time in both audible and machine readable time code form that can be used as references for radio clocks and radio-controlled watches. The audio portions of the WWV and WWVH broadcasts can also be heard by telephone. The time announcements are normally delayed by less than 30 ms when using land lines from within the continental United States, and the stability (delay variation) is generally < 1 ms. When mobile phones are used, the delays are often more than 100 ms due to the multiple access methods used to share cell channels. In rare instances when the telephone connection is made by satellite, the time is delayed by 250 to 500 ms. To hear these broadcasts, dial (303) 499-7111 for WWV (Colorado), and (808) 335-4363 for WWVH (Hawaii). Callers are disconnected after 2 minutes. These are not toll-free numbers; callers outside the local calling area are charged for the call at regular long distance rates.
Loran-C time signals may also be used for radio clock synchronization, by augmenting their highly accurate frequency transmissions with external measurements of the offsets of LORAN navigation signals against time standards.
Commercial broadcasts 
As radio receivers became more widely available, broadcasters included time information in the form of voice announcements or automated tones to accurately indicate the hour. The BBC included time "pips" in its broadcasts from 1922.
In the United States, many information-based radio stations (full-service, all-news and news/talk) also broadcast time signals at the top of the hour. In New York, WCBS and WINS, as well as WBEN in Buffalo, New York, have distinctive top-of-the-hour tones, though the WINS signal is only approximate (several seconds error) . WINS and WBEN also have a tone at 30 minutes past the hour for those setting their clocks.
Stations using iBiquity Digital Corporation's "HD Radio" system are contractually required to delay their analog broadcast by about eight seconds so it remains in sync with the digital stream. Thus, network-generated time signals and service cues will also be delayed by about eight seconds. Local signals may also be delayed.
The all-news radio stations of the CBS Radio Network, of which WCBS is the flagship, air a "bong" (at a frequency of 440 Hz, the standard musical note A) that immediately precedes each top-of-the-hour network newscast. (The same bong could be heard on the CBS Television Network, at the top of the hour immediately before the beginning of any televised program, in the 1960s and 1970s.) An automated "chirp" at one second before the hour signals a switch to the radio network broadcast. As an example, KNX, the CBS Radio Network all-news station in Los Angeles, broadcasts this "bong" sound on the hour. However, due to buffering of the digital broadcast on some computers, this signal may be delayed as much as 20 seconds from the actual start of the hour (this is presumably the same situation for all CBS Radio stations, as each station's digital stream is produced and distributed in a similar manner).
In Canada, the national English-language non-commercial CBC Radio One network broadcasts the daily National Research Council Time Signal at 1pm Eastern Time. Its French-language counterpart, Radio-Canada, broadcasts a similar signal at noon. Vancouver radio station CKNW also broadcasts time signals, using a chime every half-hour.
In Australia, many information-based radio stations broadcast time signals at the top of the hour and a speaking clock service is also available. However, the former VNG dedicated time signal service has been discontinued.
See also 
- American Practical Navigator
- Atomic clock
- Car radio, Radio Data System (RDS)
- Extended Data Services and PBS
- Global Positioning System#Timekeeping
- Greenwich Time Signal
- Low frequency, (LF)
- Marine chronometer
- Network time protocol
- Radio clock
- Radio VNG - Australia
- Radio-controlled watch
- Smart Personal Objects Technology, (SPOT)
- Speaking clock
- Time transfer
- VBI, VITC, Time code
- Aubin, David The Heavens on Earth: Observatories and Astronomy in Nineteenth-Century Science and Culture p.164. Duke University Press, 2010
- Greenwich Time Ball
- R. W. Burns, Communications: an international history of the formative years, IET, 2004 ISBN 0863413277 page 121
- Dennis D. McCarthy, P. Kenneth Seidelmann Time: From Earth Rotation to Atomic Physics,Wiley-VCH, 2009 ISBN 352740780, page 256
- Dennis D. McCarthy, P. Kenneth Seidelmann, Time: From Earth Rotation to Atomic Physics , Wiley-VCH, 2009 ISBN 3-527-40780-4, page 256
- iBiquity Licensing Fact Sheet 2009 (PDF).
- Downing, Michael, Spring Forward: The Annual Madness of Daylight Saving Time (Shoemaker and Hoard, 2005) ISBN 1-59376-053-1
- "The Time Ball and the One O'clock Gun". Proudman Oceanographic Laboratory, Liverpool, UK. 2007. Retrieved 2 June 2008.
- Travel Downunder mentions the time cannon at Perth
- Durr Cannon Research Projects: mentions the time cannon at Cape Town
- The VNG User's Consortium is working on ways to solve the problem of the lack of accurate time signals in Australia.
- The Edinburgh time gun
- Distribution of Time Signals by the U.S. Naval Observatory
- Distributed Time Service, an upgrade to current time station technology
- Receiving, identifying and decoding time signals by Nick Hacko, VK2DX