In the US (and a few other places) the 24 hour clock is not commonly used or understood by the average person. When context cannot distinguish between two similarly numbered hours from different parts of the day, AM or PM is typically appended. However, if you find AM/PM confusing, then the following might help when communicating with others who are not familiar with the 24 hour clock. It is also handy for reading any times written this way (e.g. plane, train or bus times).

The short version

• Consider the day divided into two, equal, twelve hour chunks, the first called ‘AM’ and the second ‘PM’.
• It might help to think ‘A is before P‘ [alphabetically], if you struggle to recall the order.
• These sections start with ‘12’ as the zero hour, thus: 12AM, 1AM, 2AM … 10AM, 11AM; followed by 12PM, 1PM, 2PM … 10PM, 11PM
• When talking about the ’12:00′ hours themselves, it is preferable to avoid the AM/PM labels altogether and just say ’12 midnight’ or ’12 noon’ (midday).

To expand on that last point, if you need to say 12 noon plus some arbitrary amount of minutes, say something like, ’12:30 in the afternoon’. For times immediately following midnight, things like ’20 minutes past midnight’ (00:20) or ‘quarter to 1 in the morning’ (00:45) also work well and should be clearly understood by the majority of people.

Why AM/PM is difficult

AM/PM is confusing for a couple of reasons. Firstly while common in the English speaking world, the acronym is not in English but rather in Latin, thus making it hard for people to recall. Secondly the actual meaning makes no logical sense in relation to the ’12’ hours.

AM = Ante Meridiem (before noon [midday])
PM = Post Meridiem (after noon [midday])

So here is the problem in a nutshell. At exactly 12:00 midnight you could arguably be both AM or PM depending on which noon you refer to (the previous or the forthcoming), since they are equidistant from a time perspective. While at exactly 12:00 midday, you are definitively neither AM nor PM because it is precisely ‘noon’.

The most common way to write these is with midnight as 12AM and noon/midday as 12PM†. The reason for this is that while 12:00 midday is neither before or after noon, the rest of the minute and second subdivisions of that hour are clearly PM (after noon) and thus you might as well give 12:00 midday the PM label as well, to keep it in same group. It’s not logically, 100% correct but it is the least bad choice between AM and PM when neither really work. Similarly 12 midnight cannot be categorised cleanly but 12:11 is obviously AM (before noon) in relation to the closest noon (the one happening on the same day). So again midnight as 12AM is arguably the least bad option.

I think it is worth knowing the AM/PM time notation if you interact with people from the US (or others that use this system) or if you are planning a holiday there. It helps to be able to read things like timetables or digital clocks that use the AM/PM notation. However I would strongly suggest you avoid saying AM/PM alongside ’12’ when talking to other people. The point here after all is to be more precise, not more ambigious, yet you will occasionally find people who have grown up with AM/PM notation who have a different interpretation of the correct label for the ’12’ hours. That might be because they learnt the meaning and found another argument to favour the other option, or maybe because it just ‘feels weird’ to think of 12 as the starting hour. No matter, if you simply say ’12 noon’ or ’12 midnight’ you should be understood.

AM/PM versus the 24 hour clock

I just wanted to end this stating, this is not a promotional piece trying to encourage the use of AM/PM. I personally prefer the 24 hour clock. The post is just to give tips on how to read and express time with AM/PM in case you ever needed to. That is all, don’t read into it more than that or accuse me of promoting some AM/PM agenda! 😜

† Some people will state I am wrong and indeed you can easily find references to midnight being labelled as 12PM online 😆. I am just stating the ‘most common’ way to interpret these two times, so you can have some level of confidence when reading a railway timetable. And in all honesty it does not matter if you actually agree with me. This just re-emphasises the point that you should avoid using AM/PM at all when talking about ’12’, if you can. 😉