A child is said to be a year old on the 365th day, after the beginning of the day, not at the end of it, because we reckon the civil year not by moments, but by days.
Definition of Anniculus Trecentesimo Sexagesimo Quinto Die Dicitur Incipiente Plane Non Exacto Die Quia Annum Civiliter Non Ad Momenta Temporum Sed Ad Dies Numeramur
Definitions from Black's Law Dictionary: 2nd Edition and Ballentine's Law Dictionary as are available for each term in each dictionary.