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This means that by counting the number of daily bands per year, we can find out how many days there were per year at the time when they were formed.
In the same way, by looking at mussels we can find out how many low tides there were per lunar month when they grew.
If a change of one day per year corresponds to the passage of 10 million years, then this limits the precision with which we can resolve the age of a shell or coral.
What is more, the change in day length is not as predictable as the decay of radioactive isotopes.
Such things as the position of the continents and of mid-ocean ridges will affect tidal patterns, and these change over time as we have seen in our discussion of plate tectonics.
Then again, the formation of polar ice-caps, and the concomitant fall of sea-levels would speed up the Earth's rotation as a consequence of the law of conservation of angular momentum.
The friction of the tides slows down the Earth's rotation: this is known as tidal braking.
The effect, though small, is measurable by the high-precision clocks used by astronomers, and so can be established directly as well as on theoretical grounds: at present, the effect amounts to a day getting longer by 2.3 milliseconds over the course of a century (see here for more details).
We should note that in fact scientists do not simply extrapolate the present rate of slowing in a linear manner to calculate past rates of rotation, but rather calculate this from the physics of the Earth-Moon system.
In the same way, mussels deposit their growth bands every low tide, but also show variations according to the phase of the moon, so that they keep count both of low tides and of lunar months.
The photograph above right shows the clam Arctica islandica, a popular species with sclerochronologists: growth bands are visible in the shell.
It is possible to use these growth patterns to date recent shells (and so the sediments that contain them) in a manner analogous to dendrochronology.
However, there is a more interesting way of using this data, which we shall discuss in the remainder of this article.
For the purposes of this article, it is not necessary to go into the details of the calculation.