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Comment by mauvehaus | Hacker News Reader

The way to remember this that finally clicked for me is that 'A' means 'ante', 'P' means 'post', and 'M' means Meridian some Latin word that means the sun is directly overhead. Or more practically speaking: noon.

12:00 PM is noon because one minute later, the sun is past being directly above. That is, it's 12:01 Post Meridiem. Since it would obviously be nonsensical for 12:00 AM to be immediately followed by 12:01 PM, noon is 12:00 PM by convention.

Extending this logic to midnight is left as an exercise to the reader.

Don't feel bad, I don't think anybody explained this to me until I was almost 30.

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Does not make any sense at all.

You jump from "sun is overhead in 12 hours" (12 AM) to "sun is overhead in 1 hour" (1 AM) when you follow that logic.

The 12 hour-system is broken and should go away.

Would you want the clock face to be changed to show 24 hours instead of 12? (What would be the number at the top then - 0 or 24?)

Easy alternative: start the clock at zero like a rational individual would. 12 only makes sense if you see it as the end and go backwards from there. It's a bit like the confusing mess of how year numbers work where traditionally there is no year zero (because the year one is 1 AD, i.e. the first year "of our Lord", i.e. year numbers are actually ordinal even though everyone treats them as nominal).

Besides, at this point 90% of the time formats people look at in their daily lives are probably digital (since analog faces are only relevant to actual clocks and even most clocks are likely digital simply because so many electronic devices come with digital clocks) and even analog clocks don't always have numbers on them (or use fanciful alternatives like Roman numerals). The people who prefer analog clocks are clearly just sticking with them out of personal preference or aesthetics anyway.

You're making it sound as though a 24-hour format is a wild new idea when it's actually in common use across 100+ countries. Timestamps go from 00:00:00 to 23:59:59 without breaks in continuity. The clock still has 12 hours with the 12 on top, but it is commonly understood that "12" on the clock face can refer to either 0 or 12, just like how "5" can refer to either 5 or 17.

What’s the difference? In both the 12 and 24 hour system the clock rolls over at the end of the period, and the period starts.

The difference is that in 12-hour system, the rollover point is ambiguous and non-intuitive (11 PM -> 12 AM -> 1 AM), while in 24-hour one it's perfectly clear and obvious.

The 24 hour system: 23 -> 0 -> 1

How is that different than 11 -> 12 > 1 ?

Yes I get it that in normal integers, 1 is not after 12, but neither is 0 after 23.

You're missing the most important bit - the one this whole thread is about.

The 24 hour system: 23 -> 0 -> 1

The 12 hour system: 11 PM -> 12 AM -> 1 AM

In the 24 hour system, there's a single point where the new day starts, and it's the transition from 23 to 0.

In the 12 hour system, where does the new day start? Numbers tell you it's 12 -> 1, but it's actually 11 -> 12. You have no way to figure the answer to OP's question out without additional context.

As others already pointed out in this thread, a 12 hour system using 0 instead of 12 would make it much more intuitive, although now you'd end up with either using 0:00 PM for noon, which feels strange, or using 0:00 AM and 12:00 PM, which is inconsistent. The 24 hour system has no such issues and has a massive advantage over creating new systems - people are already familiar with it.

> "where does the new day start?"

I honestly am trying to understand the point being made. Isn't when the day starts obvious in either case? We know when the new _day_ starts because the date rolls over at that time. No one thinks the day starts at 12:00 in the 24-hour system, or 12:00pm / noon in the 12-hour system?

> ... there's a single point where the new day starts ...

Is this what you're saying: that in the 24 hour system, there's one point in time where the clock resets from a high number to zero, and that's also when the day starts. Therefore, it is "not confusing."

I felt rather that the problem the OP stated was simply "I am used to the 24 hour system and I was communicating with someone who was used to the 12 hour system. They naturally gave me a time of 12:00am but I didn't notice that am part and assumed they meant the same "12" that I understand as "12" in my system. So I missed an exam."

> We know when the new _day_ starts because the date rolls over at that time.

That's an additional reasoning you have to do in order to explain the non-intuitive fact that the new day starts when transitioning between hour 11 and 12. You can't come up with that conclusion based on clock system alone, you need to reach for the calendar. It makes you jump additional mental hoops, making it error prone (as evidenced by this thread).

> but I didn't notice that am part and assumed they meant the same "12" that I understand as "12" in my system

No, people who use 24 hour system are usually familiar with 12 hour system as well (guess what's there on analog clocks?). It's just this one special case of 12 AM/PM that's confusing, because you can make convincing arguments for having it both ways.

Well, some of them sound more convincing than the others. Intuitively, "12:30 AM" (whose meaning is unambiguous) seems to have more affinity to 12:00 than 11:30 PM does.

But since the midnight is the same distance from the noon, before (AM) or after (PM), in theory both should be reserved for midnights (the past one and the future one), and noon, too, should be both AM and PM - at 0 hours.

The only real solution for this mess is the one that everyone uses: just burn the meaning of 12:00 AM as midnight into your brain, and those who fail to do so are in for a trouble.

There's a much better solution used by a huge chunk of the world already - the 24 hour system, which avoids these pitfalls completely. "Burning the meaning into your brain" is just destined to fail when you use the 12 hour system rarely enough, like a lot of people do.

I just don't see what you're seeing I guess. The clock starts at 00:00 and goes from 23:59 to 00:00 in the 24 hour system. In the 12 hour one, it always starts at 12:00 and goes from 11:59 to 12:00. That parts not confusing at all.

I do understand that having two different 12's in a day introduces an ambiguity about which one is the "real start" of the day just based on "12:00" alone.

I find the 12-hour system as intuitive as the 24-hour system. It is centered around noon, where "AM" means time on the same day before noon, and "PM" means time after, on the same day. Nothing wrong with that.

That's only because you're used to it and have additional context already. The day starts at 12 and goes to 1 an hour later, so notion of "same day" is absolutely ambiguous.

To clarify, I was responding to the "should go away" part.

Uh, doesn't 1:00 AM mean the sun is overhead in 11 hours?

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