Original: The year of his death is calculated from the fact that Antonio Pucci, the town crier of Florence, wrote a poem in Giotto's honour in which it is stated that he was 70 at the time of his death.
Simplified: The year of his death is known, but the year of his birth is based on a poem by Antonio Pucci, the town crier of Florence, who said that Giotto was seventy when he died.