Oh, Lord. We've heard of mistakes but this one is truly Great. According to the Pope, the Christian calendar is years off the mark - because of one monk's bad maths.
In his new biography of Christ, Pope Benedict XVI writes that a sixth century monk known as Dennis The Small - or Dionysius Exiguus - got it wrong, meaning that Jesus was actually born several years earlier than it is previously thought, the Daily Telegraph reports.
"The calculation of the beginning of our calendar – based on the birth of Jesus – was made by Dionysius Exiguus, who made a mistake in his calculations by several years," he writes.
"The actual date of Jesus's birth was several years before."
Jesus of Nazareth: The Infancy Narratives is the third instalment of the Pope's biography of Jesus.
According to the BBC, the Pope's spokesperson Father Federico Lombardi said the pontiff had dedicated "all his free time to bring to fruition this project, which he wanted and loved."
The Daily Mail reports that the Pope has even questioned the place of the donkey in the nativity scene.
"In the gospels there is no mention of animals," he says.
But don't fear, we won't be seeing an animal free nativity scene in the future - with the pope adding: "No nativity scene will give up its ox and donkey."
John Barton, Professor of the Interpretation of the Holy Scripture at Oriel College, Oxford University told the Telegraph most agreed with the pope.
"It's been surmised for a very long time that Jesus was born before 1AD - no one knows for sure," he said. "We don't even know which season he was born in. The whole idea of celebrating his birth during the darkest part of the year is probably linked to pagan traditions and the winter solstice."