When talking about time, a.m. (or am) stands for 'ante meridiem', which is Latin for 'before midday' and p.m. (or pm) stands for 'post meridiem', which is Latin for 'after midday'.
When we use noon or midday it’s quite clear which day we are talking about, but midnight depends upon the context. For example, when we say: 'Bookings can only be made between 10am and midnight', midnight refers to the same day. But in the statement: 'From midnight on the first of the month', midnight belongs to the following day.
Similarly, 12 noon and 12 midnight are clear, but the terms 12 am and 12 pm can be confusing, because there is no agreed standard and they are used in different ways.
By convention, 12 am refers to midnight at the start of the specified day (00:00 on the 24 hour clock) and 12 pm to midnight at the end of that day (24:00 on the 24 hour clock). It can be argued that by the time you have seen a clock showing 12:00 at midnight it is already ante meridiem, and similarly at midday it is already post meridiem. You can see that the 24-hour clock is much clearer.
12 noon is, by definition, neither ante meridiem (before midday) nor post meridiem (after midday). For clarity, times in the first hour of the day are sometimes given with am, such as 00:47 am, and the time twelve hours later given as 12:47 pm.
To avoid ambiguity, it is better to use the 24-hour clock, when 12:00 is always 12 noon, and 24:00 Sunday or 00:00 Monday both mean 12 midnight between Sunday and Monday. It is common in transport timetables to use additional information on the day or am or pm to reduce confusion.
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