High Quality Content by WIKIPEDIA articles! The 12-hour clock is a time convention in which the 24 hours of the day are divided into two periods:a.m. (from the Latin ante meridiem, meaning "before midday") and p.m. (post meridiem, "after midday"). Each period consists of 12 hours numbered: 12 (acting as zero),1, 2, 3, 4, 5, 6, 7, 8, 9, 10, and 11.