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, meaning past midday). Each period consists of 12 hours numbered: 12 (acting as zero), 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, and 11. The 24 hour/day cycle starts at 12 midnight (often indicated as 12 a.m.), runs through 12 noon (often indicated as 12 p.m.), and continues to the midnight at the end of the day.
It is not always clear what times “12:00 a.m.” and “12:00 p.m.” denote. From the Latin words meridies (midday), ante (before) and post (after), the term ante meridiem (a.m.) means before midday and post meridiem (p.m.) means after midday. Since “noon” (midday – meridies (m.)) is neither before nor after itself, the terms a.m. and p.m. do not apply. The American Heritage Dictionary of the English Language has a usage note on this topic: “By convention, 12 AM denotes midnight and 12 PM denotes noon. Because of the potential for confusion, it is advisable to use 12 noon and 12 midnight.”