What is the Difference Between 12:00 AM and 12:00 PM?

[arydberg]

Summary:: what do 12:00 AM and 12:00 PM really mean.

How do you say the time at at noon. is noon 12:00 AM or 12:00 PM? 12:00 AM means 12 hours before meridian or before noon so 12:00 AM should mean 12 hours before noon or midnight.

12:00 PM means 12 hours past meridian or 12 hours past noon or also midnight. None of this makes sense but computers seem to have a problem with times like noon and midnight.
So what is going on?

The same argument applies to midnight. How do you say the time at midnight.

 

[PeroK]

You can say 12 noon and 12 midnight. That's what I do.

https://www.rmg.co.uk/stories/topics/noon-12-am-or-12-pm

 

[Ibix]

The argument I heard is that noon is exactly 12:00:00.000000... But by the time the light has reached your eye from your clock it's after that, so 12pm is (just after) the middle of the day. Similarly, 12am is (just after) the middle of the night.

I prefer noon and midnight, as PeroK does, but the above helps me sort it out when some service or person is talking to me about 12pm.

 

[berkeman]

The ambiguity is why we use military time in EMS and in Radio communication. It takes a little getting used to (not much), but the benefits are worth it. Midnight is 0000 (zero hundred hours) and noon is 1200 (12 hundred hours).

It also helps a lot when you are working an overnight medical shift when Daylight Savings Time kicks in ("Fall Back") at 0200 Sunday morning. I can fill out a Patient Care Report with a Start Time of 0100 PST, and another one two hours one hour later at 0100 PDT and both PCRs are correctly timestamped.

 

[Vanadium 50]

I can fill out a Patient Care Report with a Start Time of 0100 PST, and another one two hours later at 0100 PDT and both PCRs are correctly timestamped.

Two hours? The Pacific does Double Daylight Savings Time?

 

[berkeman]

Two hours? The Pacific does Double Daylight Savings Time?

Oh, woopsies, one hour later. Doh! (fixed)

 

[jack action]

12:00 AM means 12 hours before meridian or before noon so 12:00 AM should mean 12 hours before noon or midnight.

How does that theory work with 1:00 AM?

If noon would be 12:00 AM then 12:30 AM would be after the meridian, which doesn't make sense.

12:00 PM -> Noon; 12:30 PM -> 30 minutes after noon, after the meridian
12:00 AM -> Midnight; 12:30 AM -> 30 minutes after midnight, before the meridian

 

[rsk]

It also helps a lot when you are working an overnight medical shift when Daylight Savings Time kicks in ("Fall Back") at 0200 Sunday morning. I can fill out a Patient Care Report with a Start Time of 0100 PST, and another one two hours one hour later at 0100 PDT and both PCRs are correctly timestamped.

When you fly from Spain to Portugal you arrive before you left.

 

[symbolipoint]

Summary:: what do 12:00 AM and 12:00 PM really mean.

This I have not checked but believe it to be the way things are used:
Midnight is 12AM, and Noon is 12PM.

 

[robphy]

I have had students tell me that
they missed a 12noon deadline
because
they thought
“12:00pm” meant midnight, as the minute after 11:59pm.

…and similar confusions for 12midnight.I think a recent update in our course management software now gives “11:59pm” as a selectable option for due date time.

 

[DennisN]

what do 12:00 AM and 12:00 PM really mean.

I don't know, I never took the time to find out.

Here in Sweden we use a 24 hour format; the clock goes from 0:00 (midnight) to 12:00 (noon) and then onwards to 23:59 which is one minute before midnight and then the clock resets to 0:00.

I remember having trouble remembering the AM and PM format, but I made a personal memory trick:

"AM is After Midnight."
"PM is 'På Morgonen'.", which means "in the morning" in Swedish, so it's not a useful memory trick in English.

Edit: Haha, it was a long time since I did this memory trick, and I realized now that it's not so good, since PM is not morning. But it has worked for me.

Maybe a good replacement memory trick could be "PM is Past Morning." (not perfectly accurate, though)

 

[JT Smith]

The ambiguity is why we use military time in EMS and in Radio communication.

It's not ambiguous. There is a convention. It's just not particularly logical.

Why does Tuesday come after Wednesday instead of before it?

 

[symbolipoint]

It's not ambiguous. There is a convention. It's just not particularly logical.

Why does Tuesday come after Wednesday instead of before it?

You're taking a poke at what is and isn't logical, so to be humorous. The community who gave use the names of the week used names of gods and were not trying to use alphabetical order. Otherwise, we have some work to do with renaming the days of the week. Then again, which language to use for naming the days makes this more troubling.

 

[darth boozer]

"12:00 AM means 12 hours before meridian"

This cannot be correct. 11:00 AM is only 1 hour before noon, not 11.
The 12-hour clock is a time convention in which the 24 hours of the day are divided into two periods: a.m. (from Latin ante meridiem, translating to "before midday") and p.m. (from Latin post meridiem, translating to "after midday"). Each period consists of 12 hours numbered: 12 (acting as 0), 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 and 11. The 24-hour/day cycle starts at 12 midnight (usually indicated as 12:00 a.m.), runs through 12 noon (usually indicated as 12:00 p.m.), and continues just before midnight at the end of the day.

 

[symbolipoint]

"12:00 AM means 12 hours before meridian"

This cannot be correct. 11:00 AM is only 1 hour before noon, not 11.
The 12-hour clock is a time convention in which the 24 hours of the day are divided into two periods: a.m. (from Latin ante meridiem, translating to "before midday") and p.m. (from Latin post meridiem, translating to "after midday"). Each period consists of 12 hours numbered: 12 (acting as 0), 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 and 11. The 24-hour/day cycle starts at 12 midnight (usually indicated as 12:00 a.m.), runs through 12 noon (usually indicated as 12:00 p.m.), and continues just before midnight at the end of the day.

The part of the naming, "o'clock" has some meaning here.
Eleven AM is 11:00 of the clock, but before midday. The full day is cut into two cycles of 12-hours each.

 

[darth boozer]

The part of the naming, "o'clock" has some meaning here.

The point in question was the OP's statement about what a 12:00 AM means, as opposed to what it represents.
To say that 12:00 AM means 12 hour before midday (replacing "meridian"!) implies that 11:00 AM would mean 11 hours before midday, which it clearly does not. There was no mention of "o'clock".

 

[symbolipoint]

The point in question was the OP's statement about what a 12:00 AM means, as opposed to what it represents.
To say that 12:00 AM means 12 hour before midday (replacing "meridian"!) implies that 11:00 AM would mean 11 hours before midday, which it clearly does not. There was no mention of "o'clock".

How do you find that?

 

[Klystron]

The ambiguity is why we use military time in EMS and in Radio communication. It takes a little getting used to (not much), but the benefits are worth it. Midnight is 0000 (zero hundred hours) and noon is 1200 (12 hundred hours).

Here in Sweden we use a 24 hour format; the clock goes from 0:00 (midnight) to 12:00 (noon) and then onwards to 23:59 which is one minute before midnight and then the clock resets to 0:00.

Twenty-four hour time format not only reduces problems and ambiguities, the format suits modern technological life. One cannot expect an air traffic control operator to open a window to confirm that the sun has indeed set for a 09:00 PM handoff. The simple format 2100 though local contains the necessary information with minimal symbols without ambiguity.

While I became accustomed to 24 hour format while serving in the military, I prefer '24-hr. clock' to the term 'military time'. My sister learned the identical format in nursing school. We both continue to use 24-hr. time format in favor of the antiquated AM/PM form.

Computers use several time hacks, none AFAIK requiring AM or PM. Some systems simply increment a positive integer once each time interval beginning at an arbitrary date/time. This value can be compared and synchronized with an authoritative time stamp broadcast from the Naval Observatory in North America or similar regional authority, corrected for transmission lags. Many computer owner/operators prefer Coordinated Universal Time (UTC) format. On screen time displays follow user preferences.

This issue "What time is it?" remains far from trivial. I once helped a lab director prepare for a lawsuit requiring emails archived before I joined the organization. Email arrival, opening and archive times while central to the lawsuit were far from clear. IMS case dismissed from lack of evidence.