Midday Midnight Time?

[Magic 1]

What time is Midnight 12am ? What time is Midday 12pm ? or is there no such time ?

[Clanad]

AM and PM start immediately after Midnight and Noon (Midday) respectively.
This means that 00:00 AM or 00:00 PM (or 12:00 AM and 12:00 PM) have no meaning.
Every day starts precisely at midnight and AM starts immediately after that point in time e.g. 00:00:01 AM.
To avoid confusion timetables, when scheduling around midnight, prefer to use either 23:59 or 00:01 to avoid confusion as to which day is being referred to.
It is after Noon that PM starts e.g. 00:00:01 PM (12:00:01)
Additionally  AM is abbreviation for ante meridiem
PM is abbreviation for post meridiem...
 
 

[Quizmonster]

As Clanad says, AM and PM are abbreviations of 'ante meridiem' and 'post meridiem' which mean - in Latin - 'before' and 'after the middle of the day'.

Some people apparently use 12 AM to mean midnight and 12 PM to mean midday.  However, there really is no 12 AM or 12 PM.  Why? Since 12 noon actually is the middle of the day, how could it possibly be either 'before' or 'after' itself? The same applies to 12 midnight, which is equidistant from noon, whichever way around you look at it, so it's no more before noon than it is after noon!

[Hippy]

Much better to use the 24 hour clock, no such possible ambiguity arises. Days start and end at precicely 0000hrs and midday is 1200hrs. Timetables and other synchronisations can be carried out seamlessly, and you can perform arithmetic on the time a lot easier in 24hr mode. Our friendly neighbours in France, Belgium, Netherlands, Germany, etc. have been doing this for at least a couple of generations.

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[magicdice]

However, there really is no 12 AM or 12 PM.  Why? Since 12 noon actually is the middle of the day, how could it possibly be either 'before' or 'after' itself?

but noon & midnight themselves actually last for one single instant. therefore 12pm, as it generally refers to a tangible period of time, is noon because 12 hours and 0.000000000000001 seconds after midnight is after noon. hence the PM for midday and the AM for midnight - it's a convention because even if it was AM for one instant at exactly 12 noon, it'd then flip to PM for the remainder of the hour

[Quizmonster]

Well, don't just take my word for it. How about the view of a British scientific website dealing with Greenwich and matters of time? To see what that is, click http://www.greenwichmeantime.com/info/noon.htm

If you'd prefer an American viewpoint, how about that of the American Government's National Institute of Standards and Technology? You'll find that by clicking
http://www.physics.nist.gov/News/Releases/questions.html

[magicdice]

to quote from your link, AM starts directly after midnight, i.e. to all intents and purposes actually at the instant at which midnight occurs. so my explanation was right, yes?

[Quizmonster]

Well, personally, MD, I don't think your explanation is right, but - from here on - I can't be bothered arguing about it. It seems to me that - no matter how long you deem midday or midnight to "last" - it still, by definition, cannot be either before or after itself.

In your own latest answer, you say (quote): "AM starts directly after midnight" which cannot possibly mean the same thing as "at the instant at which midnight occurs", which you tack on to it. 

What you're saying is exactly the same as saying: "Midnight happens before AM starts"? And that's precisely my point, Clanad's point and the point of the two authoritative web-pages I provided links for. For me...here endeth the story.

[slimfandango]

It is technically possible to have a precise midday or midnight only about once every thousand million billion years. This is because it is highly likely that time, like energy, is quantized, i.e. it comes in distinct 'real' indivisible units, and is not just a 'continuous goo' which you can chop up where you like. Sure, you can add arbitrary approximations like hours, minutes, and seconds, but to fall exactly on midnight you would have to have come to the last of the quantized units of time in one day and be about to enter the first of the quantized units of time the next day. The quanta are so small that only extremely rarely do our 'minute' approximations and the quanta overlap.

[slimfandango]

magic, there is no such thing as 'instant' in physics because there is no justification for picking a preferred frame of reference (cf Monsieur Einstein). The time that passes between 12pm and 2pm is two hours and can be spoken about meaningfully, but that does not mean that there was an 'instant' at 12pm, it means nothing to say this (and if we say that we are 'arbitrarily' assigning an instant to it then we can really 'arbitrarily' assign meaning to anything). Of course, these times also need to be related to space and the speed of light and whether or not things are being observed, but that's another story.