Tree rings are not quite the only system that promises total accuracy to the nearest year. Varves are layers of sediment laid down in glacial lakes. Like tree rings, they vary seasonally and from year to year, so theoretically the same principle can be used, with the same degree of accuracy. Coral reefs, too, have annual growth rings, just like trees. Fascinatingly, these have been used to detect the dates of ancient earthquakes. Tree rings too, by the way, tell us the dates of earthquakes. Most of the other dating systems that are available to us, including all the radioactive clocks that we actually use over timescales of tens of millions, hundreds of millions or billions of years, are accurate only within an error range that is approximately proportional to the timescale concerned.
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