How old is the universe? Scientists say it's around 14 billion years; young-earth creationists, perhaps around 10,000 years.
For Archbishop Ussher, however, today's the day: because according to him, the world was created on October 23, 4004 BC.
Ussher was the Archbishop of Armagh in Ireland during the most turbulent period of modern British history, the terrible Civil War. He was a fervent anti-Catholic Protestant, though a Royalist who supported King Charles; he watched his execution, but fainted before the axe fell.
Ussher believed the creation began on Sunday, October 23. And far from the popular belief that he arrived at his conclusion just by adding up the numbers given for different generations in the Bible, it was actually a considerable intellectual feat. Not only did he have to deal with the Bible's chronology, but he had to work out which calendar to use and even which version of the Jewish Torah to use, as they gave different figures for the number of years between the Flood and the Creation. He also had to decide when Jesus was born, settling on 4 BC, and cross-reference the dates given for Jewish kings with events known from outside the Bible.
Ussher used the Jewish calendar to establish that the Creation began in the autumn – the Jewish new year – on a Sunday near the autumnal equinox. It had to be a Sunday because on the seventh day – the Jewish Sabbath, or Saturday – God rested from his work.
After all this he used Kepler's astronomical tables to fix the final date.
Ussher wasn't the only person to try to do this, but his formidable learning gave him considerable authority. His became the generally accepted version, though, because from 1701, his dates were included in the margins of the King James Bible.
Very few Christians today would think Archbishop Ussher got it right, and most wouldn't argue with scientists over the age of the earth. However, we can sometimes learn from the mistakes of others as well as from their successes – and if Archbishop Ussher teaches us anything, it's that we should go to the Bible for what it does teach us, not for what it doesn't. As Galileo is believed to have said: 'The Bible teaches us how to go to heaven, not how the heavens go.'
Follow Mark Woods on Twitter: @RevMarkWoods