The 12-hour clock is a way of determining the time in which the 24 hours of the day are divided into two sections called ante meridiem ( a.m. ) and post meridiem ( p.m. ) . The 12-hour clock is a time conversion convention in which the 24 hours of the day are divided into two periods called ante meridiem ( a.m. , English : `` before midday '' ) and post meridiem ( p.m. , English : `` after midday '' ) .