AM and PM (can be written in lowercase, with or without dots after the letters) are two acronyms of Latin origin used to refer each of the two 12-hour periods in which the day is divided: AM (Before Meridiem) means "before midday" and PM (Post Meridiem) means "after noon".
AM is the period starting at midnight (00:00) and ends at 11:59; PM is the period starting at noon (12:00) and ends at 23:59.
In Brazil, the counting of hours is continuous after 12:00, corresponding to 13:00 to 1:00 in the afternoon, 14:00 to 2:00 in the afternoon and so on.
In most English-speaking countries it is customary to specify the hours of the day from one to twelve, followed by AM or PM depending on the period. In these countries it is always necessary to inform the period to which the indicated time refers. For example, 3:00 PM corresponds to 3:00 PM.
In English, AM may be referred to as before noon (before midday or before noon) or after midnight (after midnight). To avoid confusion about which period refers to 12 AM or 12 PM, it is important to indicate 12 midnight (midnight) or 12 noon (noon).
Is 12:00 am or pm?
Many people are unsure whether 12:00 is am or pm. On the 12-hour system, where the am and pm designation is used, there are 12:00 am and 12:00 pm. In this case, 12:00 am is midnight and 12:00 pm is noon.
Despite this, some people claim that they like the word meridem means "noon", the exact time of 12:00 (noon) cannot be considered am or pm, as it is exactly in the meridem. But when a person looks at the clock, even after a second of midday, it can already be considered 12:00 pm.
To avoid this ambiguity, many people avoid the 12 am or 12 pm designation and use the 24-hour system. Thus, 12:00 is always noon and 00:00 (or 24:00) is always midnight.