The accuracy of early clocks was around 1 part in 1000. As more accurate clocks were produced, new uses of time measurement were explored. As new uses were discovered, the need for even more accurate clocks became apparent. The specified accuracy in commercially available atomic clocks has reach 1 x 10 -12 . This unprecedented commercial accuracy is equivalent to a gain or loss of 1 second in a minimum of 400,000 years.
Frequency is determined by counting the number of cycles over the period of a second.
Therefore, the definition of a clock can be expressed as a device that counts the number of seconds occurring from an arbitrary starting time. From this definition it appears that a clock needs three basic parts. First, a source of events to be counted. This source can be labeled a frequency standard, frequency source, or time interval standard. Second, a means of accumulating these events or oscillations.