<Ol> <Li> ^ Jump up to: See Confusion at noon and midnight </Li> </Ol> <Li> ^ Jump up to: See Confusion at noon and midnight </Li> <P> 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 . The 12 - hour clock was developed over time from the mid-second millennium BC to the 16th century AD . </P> <P> The 12 - hour time convention is common in several English - speaking nations and former British colonies, as well as a few other countries . </P>

When does the clock switch from am to pm