Time sure flies when you’re not keeping track of it, yet even you do, it sometimes gets confusing when seemingly ambiguous terms get interlocked with the way we express ourselves, especially regarding the passage of time. For instance, we often hear people refer to different times of day followed by ‘AM’ or ‘PM’, but what do these abbreviations actually mean?
A brief history
Historically speaking, the natural day / night division of a day used to be expressed through one cycle which was tracked according to the position of the Sun during the day and another cycle that was tracked according to the movement of the Moon and stars across the night sky. Over time, this two-cycle system evolved into the two twelve-hour periods that most of the world uses today.
Most ancient civilizations as far as the Romans, Mesopotamia or Ancient Egypt used 12-hour clocks to keep track of time, although the Romans used to number the morning hours in reverse.
In this context, ‘reverse’ meaning that they used to express how many hours there were until noon, unlike the modern way of counting towards it. Interestingly enough, it was actually the Romans who came up with the AM and PM denotations.
Due to the fact that a 12-hour clock divides a day into two halves, each of the 12 hours of every half has to be expressed in a distinctive way. This is where the AM / PM system comes into play, making it clear as to which hour of which half is being talked about.
The meaning of AM (sometimes spelled ‘a.m.’ or ‘am’) is ‘ante meridiem’, a Latin term that translates to ‘before noon’. We already talked about how the Romans used to measure morning time taking noon as a reference point, which is why they put so much emphasis on the hours before it.
The meaning of PM (sometimes spelled ‘p.m.’ or ‘pm’) is ‘post meridiem’, meaning ‘after noon’. The 12 hours after noon until midnight are all ‘post meridiem’ but not midnight itself.
Furthermore, one should avoid using AM or PM when talking about 12 o’clock, seeing how both noon or midnight are actually the reference points of the two-cycle system we talked about.
Use by country
While some countries use the 12-hour clock in both the written and spoken form, other countries use the 12-hour clock only when talking about the time. These countries use the 24-hour notation for technical issues, but refer to the 12-hour system when talking about time.
Interestingly enough, both the AM and the PM terms are mostly used by English-speaking countries, the same countries that use phrases like ‘in the morning’, ‘in the afternoon’, ‘in the evening’, or ‘at night’ when trying to accentuate the exact hours.
On a similar note, the current international clock uses a 24-hour format, which has always been the case since its inception.
As a matter of fact, the AM/PM form is mostly used by countries that used to be part of the British Empire e.g. English-speaking countries (UK, US, Australia).
By contrast, the rest of the world uses the 24-hour format and so do most computers and electronic devices.