Geologists discovered sedimentary deposits near ancient coastlines dated from about 750 million years ago or so, and the layering sequence of the sediments could be traced in the rock, and by counting the layers during a particular dating sequence, they deduced that the Earth was rotating about once every 18 hours. There was a recent article about this by C.P. Sonett in the journal Science 1996, vol. 273, page 100. They found by studying the laminations in tidal sediment deposits from Utah, Indiana, Alabama and Australia, that some 900 million years ago the day was only 18 hours long, and there were 481 days to the 'year'.