Ussher's work, more properly known as the Annales veteris testamenti, a prima mundi origine deducti (Annals of the Old Testament, deduced from the first origins of the world), was his contribution to the long-running theological debate on the age of the Earth. This was a major concern of many Christian scholars over the centuries.
The chronology is sometimes called the Ussher-Lightfoot chronology because John Lightfoot published a similar chronology in 1642–1644. This, however, is a misnomer, as the chronology is based on Ussher's work alone and not that of Lightfoot. Ussher deduced that the first day of creation began at nightfall preceding Sunday, October 23, 4004 BC, in the proleptic Julian calendar, near the autumnal equinox. Lightfoot similarly deduced that Creation began at nightfall near the autumnal equinox, but in the year 3929 BC.
Ussher's proposed date of 4004 BC differed somewhat from other Biblically based estimates, such as those of Bede (3952 BC), Ussher's near-contemporary Scaliger (3949 BC), Johannes Kepler (3992 BC) or Sir Isaac Newton (c. 4000 BC). Ussher's specific choice of starting year may have been influenced by the then-widely-held belief that the Earth's potential duration was 6,000 years (4,000 before the birth of Christ and 2,000 after), corresponding to the six days of Creation, on the grounds that "one day is with the Lord as a thousand years, and a thousand years as one day" (2 Peter 3:8). This view had been almost completely abandoned by 1997, six thousand years after 4004 BC. Today some biblical scholars, as well as a number of literalist evangelical Christians, believe in a literal interpretation of the Bible calling for a 6000-year-old Earth.