The Moon causes tides. Tides make the Earth slightly asymmetrical, and one result is that the Earth's rotational energy is slowly being stolen by the Moon. We spin more slowly: and the Moon rises to a higher, slower orbit.
This was worked out mathematically in the 1800's. Today, however, it has been measured.
One consequence is that in the future, there will be fewer days in a year. And in the past, there would have been more.
Modern corals deposit a single, very thin layer of lime once a day. It is possible to count these diurnal (day-night) growth lines. You can also count annual growth. So, given the right piece of coral, you can measure how many days there are in a year.
These measures can equally well be done on fossilized coral. For example, coral from the Pennsylvanian rockbeds have about 387 daily layers per year. Coral from the Devonian rockbeds have about 400 daily layers per year. In the Cambrian, a year was 412 days. One Precambrian stromatolite gave 435 days per year.
With bivalves, you can count days and lunar months. Recent bivalves give 29.5 days per lunar month; Pennsylvanian give 30.2; Devonian give 30.5.
If you care, there ore a lot more details about coral. There's a reading list at the rear of that, and the topic is covered in Strahler, and the Creationist Daniel Wonderly has written about it. But there are also some broader issues.
For one, do all these numbers increase, as one goes to supposedly older and older layers of the "geologic column"? The answer is yes.
For another, are these numbers the same, if one takes corals from different continents, but in the "same" rock layer? The answer is yes.
For a third, do these numbers agree with the theoretical numbers that the astronomers had in hand? In order to tell, we need to use radioactive dating techniques, to get dates for the various rocks. So, the comparison is somewhat a test of radioactive dating.