Most digital computers depend on an internal signal at constant frequency to synchronize processing; this is referred to as a clock signal. (A few research projects are developing CPUs based on asynchronous circuits.) Some equipment, including computers, also maintains time and date for use as required; this is referred to as time-of-day clock, and is distinct from the system clock signal, although possibly based on counting its cycles.
For some scientific work timing of the utmost accuracy is essential. It is also necessary to have a standard of the maximum accuracy against which working clocks can be calibrated. An ideal clock would give the time to unlimited accuracy, but this is not realisable. Many physical processes, in particular including some transitions between atomic energy levels, occur at exceedingly stable frequency; counting cycles of such a process can give a very accurate and consistent time—clocks which work this way are usually called atomic clocks. Such clocks are typically large, very expensive, require a controlled environment, and are far more accurate than required for most purposes; they are typically used in a standards laboratory.

Most digital clocks use electronic mechanisms and LCD, LED, or VFD displays; many other display technologies are used as well (cathode ray tubes, nixie tubes, etc.). After a reset, battery change or power failure, these clocks without a backup battery or capacitor either start counting from 12:00, or stay at 12:00, often with blinking digits indicating that the time needs to be set. Some newer clocks will reset themselves based on radio or Internet time servers that are tuned to national atomic clocks. Since the advent of digital clocks in the 1960s, the use of analog clocks has declined significantly.

Howard Miller 625-613 Company Time II Large Wall Clock is a 34" wall clock with an antique nickel frame. The center dial is a bubbled panel that is heavily distressed and deeply dented for an aged appearance. The individual off-white number panels are curved and dented with Black Roman numerals centered on each panel. The accurate quartz movement requires one "AA" battery. Always measure your area to make sure the wall you have will accommodate this large of clock. One Year Warranty and Free Shipping.

About 1581 Galileo noticed the characteristic timekeeping property of the pendulum. The Dutch astronomer and physicist Christiaan Huygens was responsible for the practical application of the pendulum as a time controller in clocks from 1656 onward. Huygens’s invention brought about a great increase in the importance and extent of clock making. Clocks, weight-driven and with short pendulums, were encased in wood and made to hang on the wall, but these new eight-day wall clocks had very heavy weights, and many fell off weak plaster walls and were destroyed. The next step was to extend the case to the floor, and the grandfather clock was born. In 1670 the long, or seconds, pendulum was introduced by English clock makers with the anchor escapement.
Electric clocks that are powered from the AC supply often use synchronous motors. The supply current alternates with a frequency of 50 hertz in many countries, and 60 hertz in others. The rotor of the motor rotates at a speed that is related to the alternation frequency. Appropriate gearing converts this rotation speed to the correct ones for the hands of the analog clock. The development of electronics in the 20th century led to clocks with no clockwork parts at all. Time in these cases is measured in several ways, such as by the alternation of the AC supply, vibration of a tuning fork, the behaviour of quartz crystals, or the quantum vibrations of atoms. Electronic circuits divide these high-frequency oscillations to slower ones that drive the time display. Even mechanical clocks have since come to be largely powered by batteries, removing the need for winding.
Greek astronomer Andronicus of Cyrrhus supervised the construction of the Tower of the Winds in Athens in the 1st century B.C.[11] The Greek and Roman civilizations are credited for initially advancing water clock design to include complex gearing, which was connected to fanciful automata and also resulted in improved accuracy. These advances were passed on through Byzantium and Islamic times, eventually making their way back to Europe. Independently, the Chinese developed their own advanced water clocks(水鐘)in 725 A.D., passing their ideas on to Korea and Japan.
An alarm clock (alarm for short) is a clock that is designed to alert an individual or group of individuals at specified time. The primary function of these clocks is to awaken people from their night's sleep or short naps; they are sometimes used for other reminders as well. Most use sound; some use light or vibration. Some have sensors to identify when a person is in a light stage of sleep, in order to avoid waking someone who is deeply asleep, which causes tiredness, even if the person has had adequate sleep. To stop the sound or light, a button or handle on the clock is pressed; most clocks automatically stop the alarm if left unattended long enough. A classic analog alarm clock has an extra hand or inset dial that is used to specify the time at which to activate the alarm. Alarm clocks are also found on mobile phones, watches, and computers.
The Howard Miller Arendal II 625-551 Wall Clock is a long version of the ever popular Arendal Quartz Wall Clock. This tall, elongated 49" high wall clock features carved accents and a turned urn finial which complement the unique style. The aged dial offers black Roman numerals and decorative black hands. The long, wooden stick pendulum features an antique-brass spun bob which swings back and forth and is battery operated like all quartz clocks. The wooden stick swinging pendulum features an antique-brass spun bob. Quartz, triple-chime Harmonic movement plays choice of full Westminster or Ave Maria chimes with strike on the hour with 1/4, 1/2, 3/4 chimes accordingly or switch to hourly only. Bim Bam chime on the hour only. One year warranty and Free Shipping.
Synchronous electric clocks do not have an internal oscillator, but count cycles of the 50 or 60 Hz oscillation of the AC power line, which is synchronized by the utility to a precision oscillator. The counting may be done electronically, usually in clocks with digital displays, or, in analog clocks, the AC may drive a synchronous motor which rotates an exact fraction of a revolution for every cycle of the line voltage, and drives the gear train. Although changes in the grid line frequency due to load variations may cause the clock to temporarily gain or lose several seconds during the course of a day, the total number of cycles per 24 hours is maintained extremely accurately by the utility company, so that the clock keeps time accurately over long periods.

With it's quality construction and stylish mid-century modern design, the 31.5" Rumi Mid Century Wall Clock is an attractive accent for the home. Crafted entirely of iron, this large wall clock features a dark brown distressed finish and large distressed gold hands. Minimalism is reflected in the clock's open pass-through design. This clock is a must-have for anyone in search of mid-century modern inspired decor.

About 1450, clockmakers working probably in southern Germany or northern Italy began to make small clocks driven by a spring. These were the first portable timepieces, representing an important landmark in horology. The time-telling dials of these clocks usually had an hour hand only (minute hands did not generally appear until the 1650s) and were exposed to the air; there was normally no form of cover such as a glass until the 17th century, though the mechanism was enclosed, and the cases were made of brass.
“I have searched for a well-designed clock for kids and was so pleased I finally made this purchase. It was definitely pricier than other options, but this one is actually well-designed, usable, and does all the functions well. My daughter loves how cute it is, and the yellow light and green light has allowed us to get more sleep in the mornings, which is priceless. … We don’t use the nap feature or white noise machine all that much, but we like the night-light and wake-up training features. The size is perfect, and it’s not too bright. Plus it has the time on it, which is perfect for my preschooler who is learning about time.”