The plot of “Silks”, one of Dick Francis’s later novels, written in collaboration with his son Felix, turns on the photograph of a new-born foal which goes on to win The Derby. The two people with the foal, a stud groom and a young vet die mysteriously, and when the vet’s brother is later murdered, the photograph is ripped from its frame and presumably destroyed. It’s the date of the stud groom’s death that enables the novel’s hero to solve the crime, for this establishes that the foal was born in December, not the following January. Since race-horses all have their official birthday on the first of January, this date of birth means that the colt was officially a four year-old when he won the Derby and therefore ineligible to run in the race. His value as a stallion and the stud-fees his owners would collect are of course determined by his status as a Derby winner.