[tz] tzfiles contain Unix epoch for the first transition time

[Paul Eggert]

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