12 P.M. - say what?

By Murray Bourne, 22 Dec 2005

I'm sure you have seen something like this:

The service will not be available from 8:00 A.M. to 12:00 P.M.

What does that end time mean? 12 noon? 12 midnight? It should mean "12 hours after the middle of the day" (P.M. stands for the Latin "post meridian"), so it must be midnight.

But what does 12 A.M. mean? (A.M. stands for "ante meridean", or "before the middle of the day".) Twelve hours before the middle of the day is also 12 midnight. So 12 A.M. = 12 P.M. = midnight. Not a good idea.

(True story) My mother was booked on a plane that was scheduled to leave at 12 P.M. on 5th January. What does that mean? It could be midnight as 5th January is starting, or midnight as 5th January is finishing, or many people take it to mean 12 noon on 5th January. My mother was there 12 hours too late and missed the plane.

Suggestion: Avoid arranging anything to start or end at noon or midnight (unless the context is very clear.) Use "noon" or "midnight" to reduce some of the confusion.

Footnote: In Singapore, they do a strange thing with TV schedules. They assume that the next day starts at 6:00 am. So the programs for today, 22nd Dec, go from 6:00 A.M. 22 Dec to 6:00 A.M. 23 Dec. It can be confusing when trying to set up DVD recordings for 2:00 A.M. The recorder date/time is correct, but the cable schedule is not.

3 Comments on “12 P.M. - say what?”

1. Michael says:

This is what they taught me at school...

12pm = Midday
12am = Midnight

This system is fundamentally flawed. As illustrated by this question:

How many hours are between 11am and 12am?
(Hint: it's not 1 hour.)

2. Alan Cooper says:

Actually, I like your idea that 12pm and 12am are both midnight!
This acutally *resolves* the ambiguity rather than creating one since 12am is clearly the midnight at the start of the day and 12 pm is the one at the end. And as you point out noon should be called neither pm nor am since it is really just 12m. But then if we wanted to be logical about it perhaps we should also ask why is 1am not called 11am since it comes 11 hours before noon?

On the other hand if we think of 12 as meaning 12+epsilon (ie just infinitesimally after 12), which it really is once it shows on the clock, then Michael's convention becomes correct.

Of course in any case it should be criminal for airlines or other such agencies not to use a 24hr clock.

3. Jaime says:

I think it is all confusing. 12 PM should be midnight 11 hours after 1 PM. Otherwise the number 12 would be before the number 1 as if we were counting backwards (12 PM before 1 PM) if 12 PM is midday as it is affirmed.

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