The twenty-four-hour system divides the 24 hours per day into two 12-hour periods, which are equal at 12 hours each. The first twelve-hour period of the day is AM or Anti Meridiem, which means before noon. The second twelve-hour period of the day is PM or Post Meridiem, which means afternoon.
There always seems to be some confusion regarding is it AM-midnight or PM afternoon? The new day begins at midnight, and analog clocks do not display day or night differences; however, the more modern digital clocks and computer programs do designate between the AM and PM settings.
AM and PM are both Latin words which stand for Anti Meridiem and Post Meridiem respectively. Anti Meridiem 'AM' simply means Before Midday while Post Meridiem means after Midday. It is common knowledge that a day is made up of 24 hours. But the use of AM and PM has led to use of two 12 hours period in a day.
Anti Meridiem is for the first 12 hours while Post Meridiem is for the second 12 hours. The first 12 hours start from midnight and run to noon and the second 12 hours start from noon run to the midnight of the second day. The use of Anti Meridiem 'AM' and Post Meridiem helps to differentiate between the hours of the day.
For example, if I say 1am, it means an hour after midnight and 3 pm mean three hours after the midday. You can use both AM and PM in their Lower case.
AM means Anti Meridian and PM means Post Meridiem. Anti Meridien means before midday and PM means after midday. This explains why the time after 12 noon is already added with PM instead. PM can be used up to midnight while AM can be used after midnight up to the minute before noontime.
There are some people who feel a bit confused with the use of AM and PM. Hopefully, the things that are mentioned above will make things clearer for everyone so that they will know what is the right one to use. Some people love using the 12-hour format when they do not understand the use of military time too.
Yes, I have given this elsewhere. The full form of a.m. is ante meridian, which means before noon. The full form of p.m. is post meridian, meaning after noon. Afternoon has been condensed into one word, but previously people found it convenient to divide daylight hours into before - what might be a lunch break or period of rest - and after, using noon as the midpoint.