The story of how an eleven-year old boy growing up in 16th century Italy loses his birthday when the Gregorian calendar replaces the Julian calendar in 1582, and how he fights to prevent this loss. The author cleverly weaves elements of the cultural and scientific milieu of the time into an engaging and intelligent tale. Tibaldos father is a medical assistant, and his sister is a midwife. Thus, the boy grows up learning about current medical practices and his fascination for medicine makes him a fast learner. Then, when Tibaldo learns that he is about to lose his 13th birthday, he determines to do something about it. The result is both amusing and informative.