Layers of ancient tidal sediment in South Australia preserve the rhythm of the tides 620 million years ago, and counting them gives a day of 21.9 hours and a year of about 400 of them.

Tides lay down thicker and thinner bands over the fortnightly spring-neap cycle, so a fossil tidal flat is a tape recording of the Moon. The Moon was closer, the tides were larger, and the Earth spun faster. It is the only direct measurement anyone has of the length of a day before there was anything to write it down with.