Steve Allen wrote:
Williams studied tidal rhythmites and found a nearly constant number of about 410 solar days per year from 2 billion to 1 billion years before present, which is about 77000 SI seconds in one day. http://onlinelibrary.wiley.com/doi/10.1029/1999RG900016/abstract
Thanks for mentioning that; I wasn't aware of this work. It appears, though, that there's still considerable uncertainty about how long the day was way back when. A recent review says that although tidal rhythmite analysis may help estimate ancient lunar orbital periods in terms of lunar days/month, estimating the length of the ancient Earth day remains uncertain because we don't know the length of the ancient lunar sidereal month. This is in contrast to something else I think you mentioned a while ago, namely the length of the day going back to about 750 BC, for which Richard Stephenson and coworkers have amassed historical eclipse records showing that our UTC-based clocks would be off by about three hours if we naively took them back to the year 0. See, for example, Sauter et al's reconstruction of the total solar eclipse of 0319-05-06 which legend says converted Mirian III of Iberia to Christianity. Longhitano SG, Mellere D, Steel RJ, Ainsworth RB. Tidal depositional systems in the rock record: A review and new insights. Sedimentary Geology 279, 2-22 (2012-11-20). http://dx.doi.org/10.1016/j.sedgeo.2012.03.024 Morrison L. The length of the day: Richard Stephenson’s contribution. Astrophysics and Space Science Proceedings 43 (2015) 3-10. http://dx.doi.org/10.1007/978-3-319-07614-0_1 Sauter J, Simonia I, Stephenson FR, Orchiston W. The legendary fourth-century total solar eclipse in Georgia: Fact or fantasy? Astrophysics and Space Science Proceedings Volume 43 (2015) 25-45. http://dx.doi.org/10.1007/978-3-319-07614-0_3