This spike in carbon 14 in the atmosphere serves as a chronometer of the past 55 years.
From the sciencedaily.com
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An accurate clock (especially used in navigation)
A marine chronometer is a clock which is precise enough to be used as a portable time standard; it can therefore be used to determine longitude by means of celestial navigation. ...
(Chronometers) A chronometer watch is a kind of watch tested and certified to meet certain precision standards. In Switzerland, only timepieces certified by the COSC may use the word 'Chronometer' on them.
A device for measuring time, such as a watch or clock
A chronometer is a high-precision watch capable of displaying the seconds and housing a movement that has been tested over several days, in different positions and at different temperatures, by an official neutral body (COSC). ...
This term refers to a precision watch that is tested in various temperatures and positions, thus meeting the accuracy standards set by C.O.S.C. in Switzerland. These watches are provided with a chronometer certifcate detailing specific test results by the C.O.S.C.
An instrument, also called a calorimeter or tintometer, for determining the color of petroleum products.
Technically speaking, all watches are chronometers. But for a Swiss made watch to be called a chronometer, it must meet certain very high standards set by the Swiss Official Chronometer Control (C.O.S.C.). ...
A highly accurate timepiece, which has been certified by the official Swiss timing bureau, COSC.