Ussher now concentrated on his research and writing and returned to the study of chronology and the church fathers. After a 1647 work on the origin of the Creeds, Ussher published a treatise on the calendar in 1648. This was a warm-up for his most famous work, the "Annales veteris testamenti, a prima mundi origine deducti" ("Annals of the Old Testament, deduced from the first origins of the world"), which appeared in 1650, and its continuation, "Annalium pars posterior", published in 1654. In this work, he calculated the date of the Creation to have been nightfall on 22 October 4004 BC. (Other scholars, such as Cambridge academic, John Lightfoot, calculated their own dates for the Creation.) The time of the Ussher chronology is frequently misquoted as being 9 a.m., noon or 9 p.m. on 23 October. See the related article on the chronology for a discussion of its claims and methodology.