For the sake of the scholarship I would like to see a modification of the current text that reads As for leap seconds, we don't know the history of earth's rotation accurately enough to map SI seconds to historical solar time to more than about one-hour accuracy; see Stephenson FR (2003), Historical eclipses and Earth's rotation, A&G 44: 2.22-2.27 <http://dx.doi.org/10.1046/j.1468-4004.2003.44222.x>. A more complete reference to the accuracy of historical Delta T is in Historical values of the Earth's clock error Delta T and the calculation of eclipses Morrison, L. V. & Stephenson, F. R. Journal for the History of Astronomy (ISSN 0021-8286), Vol. 35, Part 3, No. 120, p. 327 - 336 (2004) as can be seen at http://articles.adsabs.harvard.edu/full/2004JHA....35..327M where, in particular, Table 1 gives uncertainty estimates back nearly 3000 years. For the oldest data there is an addendum Addendum: Historical values of the Earth's clock error Morrison, L. V. & Stephenson, F. R. Journal for the History of Astronomy (ISSN 0021-8286), Vol. 36, Part 3, No. 124, p. 339 (2005) http://adsabs.harvard.edu/full/2005JHA....36..339M In addition to that, for all practical purposes, civil time prior to 1972 did not use SI seconds, but rather mean solar seconds, and for dates prior to 1955 there cannot even be an argument. Therefore in the extended range of the tz database there is an arguably unspecified point when the kind of seconds counted changes from mean solar (UT) to atomic (SI). -- Steve Allen <sla@ucolick.org> WGS-84 (GPS) UCO/Lick Observatory--ISB Natural Sciences II, Room 165 Lat +36.99855 1156 High Street Voice: +1 831 459 3046 Lng -122.06015 Santa Cruz, CA 95064 http://www.ucolick.org/~sla/ Hgt +250 m