October 23 is (in)famous as supposed earth's birthday - this date is mentioned in many textbooks retelling the life of Irish Archbishop James Ussher (1581-1656). In 1650 Ussher published a book with the title "Annales veteris testamenti, a prima mundi origine deducti" (Annals of the Old Testament, deduced from the earliest Beginning of the World), where he reconstructed the history of the world based on the bible, Egyptian and Jewish chronologies , but also research by other scholars, like John Lightfoot (1602-1675), who published his calculations in the year 1644.
The exact time given by the Ussher-Lightfoot-Chronology - October 23*, 4004 B.C., at nine o'clock in the morning** - is often ridiculed by textbooks as (we now know) a futile attempt, but at his time Ussher's calculation were based on the most reliable information available and were not intended for practical use, but as a theological guideline. For Ussher and other scholars it was important to know the age of the earth to possibly infer the time of the rapture.
[* or 6 p.m. October 22, 4004 B.C. according to the Jewish calendar, **however Ussher don´t mentions a certain time, he only states that light was created first ]
However Ussher´s calculations were not universally accepted as the only truth - there were in fact other attempts to determinate the age of the earth and many concluded that the true age was significantly older than the known human history.
JACKSON, P.W. (2006): The Chronologers´Quest. The Search of the Age of the Earth. Cambridge Press: 291
LEDDRA, M. (2010): Time Matters - Geology´s Legacy to Scientific Thought. Wiley-Blackwell Press: 269