Which is noon – 12am or 12pm?
I’ve always assumed that noon was 12am. And midnight was 12pm.
But there’s no consensus on this.
Here’s the reasoning that I use to justify my assumption.
Using the twelve hour clock the day is divided into two periods of twelve hours – those before noon and those after noon. Noon is the time at which the sun crosses the imaginary line – the meridian – that marks the middle of the day.
The twelve hours before noon – the morning – are denoted by am (for ante meridian – before noon) and the twelve hours after noon by pm (post meridian or after noon).
So, at one minute before noon the time is 11.59am, and at one minute after noon the time is 12.01pm.
That’s quite straightforward.
However, notice that one minute past noon is known as 12.01pm, with a twelve in it. This is quite a strange convention when you think about it, because it almost implies that the time is twelve hours and one minute past the meridian (noon), which it certainly isn’t. The time should perhaps better be known as 00.01pm, or one minute past noon.
This convention of retaining the number 12 for measuring times in the hour after midday is, I think, part of the cause of the confusion about whether noon is 12.00am or 12.00pm. That’s the convention though, and we’re stuck with it.
12.01pm – one minute past noon – is definitely in the afternoon, and is rightly referred to as pm.
Noon itself though isn’t in the morning or the afternoon, being the exact moment of transition between the two, and thus doesn’t strictly need a suffix of am or pm at all other than for the very important reason of distinguishing it from 12 o’clock midnight – which is why it’s so important not to get your 12.00ams and 12.00pms confused!
To me, noon should be designated 12.00am because the number twelve in the time is related to the numbers before noon, as in 9.00am, 10.00am, and 11.00am.
Times in the first hour after midday, such as 12.15pm, are designated as pm because they are firmly in the afternoon, even though their numbering convention includes the number twelve as some sort of leftover from the previous twelve hour period rather than starting afresh at zero.