AM PM explained. Is noon 12AM or 12PM? Why are there 24 hours in a day? (2023)

Is 12:00AM Midnight or Noon?

The AM PM 12 hour time format can be confusing. When is 12:00AM Tuesday? Is it midnight on Monday, Tuesday noon, or Tuesday midnight? “It’s Tuesday morning!” you may say confidently, but is it?

You would not be the first person to disagree or be confused. We asked 100 people and received three answers to this question. That’s a range of 24 hours. You might be interested to know that there are no established standards for the meaning of 12:00 AM nor 12:00 PM, but there are answers. You may think you know, but you may not be right. Around 50% of the people we asked were wrong. Confused? Alright. Set your clocks, let’s go.

30-second overview:

In this article we explain the 12 hour AM PM clock format and answer questions including:.

  • What does am pm stand for?
  • What does am pm mean?
  • Is noon am or pm?
  • What do reputable businesses, web sites and software use?
  • When is 12:00AM?
  • Midnight Confusion – When is 12:00AM Tuesday?
  • Who uses the 12 hour clock time format?
  • Why are there 24 hours in a day?

Timewatch is a developer of professional services automation software including resource scheduling, time tracking, and add-ons for Office 365 (now Microsoft 365). Timewatch has considerable expertise in time management and the 24-hour & 12-hour clock format and how it is used in software. Timewatch is an authority on time.

What does AM and PM stand for?

Let’s start at the beginning. What exactly does ‘AM’ and ‘PM’ mean? One ‘day’ totals 24 hours. Most clocks divide a day into two 12-hour periods of AM and PM

The meridiem is an imaginary semicircle that crosses the sky from due north to due south at your local noon. So AM is the morning, when the sun has not yet reached the meridiem, and PM is the afternoon, after it has passed the meridiem. There is no definition for what the time is when the sun is exactly on the meridiem, but we all call this noon or midday.

  • AM stands for the Latin “ante meridiem”, which translates to before midday. This is the time before the sun has crossed the meridian.
  • PM stands for “post meridiem” or after midday. This is when the sun has crossed the meridian.

This is great for understanding AM and PM meaning but things are about to get complicated. According to the Royal Observatory in Greenwich, London, (this is where Greenwich Mean Time or GMT originates) at exactly 12 noon, the Sun is at its highest point in the sky and directly over the meridian. It is therefore neither ‘ante’ (AM) nor ‘post’ (PM) meridiem. At 12 midnight it also neither AM nor PM.

Ways to avoid confusion with 12:00 AM / PM

One way around this conundrum is to use a different way of communicating the time. If AM and PM is confusing use a time measurement that actually has meaning. Try using ‘12 noon’ or ‘12 midnight’. This also lacks precision and is a mouthful of words. Using the 24-hour clock, or what Americans refer to as ‘Military Time’, adds precision, is easy to say, and boosts your addition and subtraction skills.

Midnight = 00:00
Noon = 12:00

Easy, huh? Not everyone uses the 24-hour clock, though.

Is 12:00 noon 12 AM or PM?

Lack of precision is fine if we are arranging a work meeting or a time to catch up for a coffee, but if we are arranging something that could happen around midnight or noon (for example a flight or train journey) we need to know exactly what time we are talking about.

Web sites, computer systems, timetables all need to be 100 percent accurate and have no ambiguity. The 24-hour military time clock solves this, but many cultures work in AM/PM, so it is helpful to have an answer to whether noon is AM or PM.

AM PM explained. Is noon 12AM or 12PM? Why are there 24 hours in a day? (1)

The AM PM definitions don’t apply to 12:00 noon because the sun is at the meridian, but we can answer what it is just before and just after passing the meridian. Just before 12:00 noon is AM and just after is PM, so if 12:00 is either AM or PM, lets try both and see which makes most sense:

  • With 12:00 noon represented as 12:00AM
    Time would progress as: 11:59AM, 12:00AM, 12:01PM. This doesn’t look right as the hour starts as AM then goes to PM.
  • With 12:00 noon is represented as 12:00PM
    Time progresses as: 11:59AM, 12:00PM, 12:01PM. This looks best as all minutes in the hour have the same PM indicator.

Hooray! We have solved the problem! Except, of course, we haven’t. We have something that makes sense, but how does this relate to general use in the web sites, software systems and services we use?

Common use of AM PM in Business, Web Sites and Software

Returning to our earlier point that there are no established standards for AM and PM, let’s look at some airport and train times as well as popular calendar software and see if they agree. We chose the busiest airport and train station in the US and popular calendar software:

  • Hatfield Airport Atlanta 12:00-12:59PM
  • NJ Transit out of Penn Station NYC uses 12:00-12:59AM
  • Every airport we looked at uses 12:00-12:59PM
  • Microsoft Outlook uses 12:00-12:59PM
  • Google Calendar uses 12:00-12:59PM
  • Digital watches use 12:00-12:59PM

Hello to New Jersey Transit! You keep being you!

Atlanta, JFK airport, Newark Liberty International Airport (and every other airline we checked), Microsoft Outlook and Google calendars all represent the hour of 12 noon to 12:59 as PM, reinforcing the answer that noon is 12:00PM and providing the joke that all NJ Transit trains from 12:00AM-12:59AM are in fact 12 hours late!

AM PM explained. Is noon 12AM or 12PM? Why are there 24 hours in a day? (2)

Outlook (and Google) use PM for noon.

Is 12:00 midnight AM or PM?

Using the same thinking as for PM, we all know that 1:00AM is in the morning. One minute before this must be 12:59 and as the sun is definitely before the meridian (ante meridien), so it is 12:59 AM. So, the early hours of the morning are 12:01-12:59AM and midnight is therefore 12:00AM. Yet, is 12:00AM Tuesday midnight on Monday or midnight on Tuesday? Is it night or is it morning?

Is 12:00AM Tuesday midnight on Monday or Tuesday?

Midnight Confusion – When is 12:00AM Tuesday

Imagine being asked to pick someone up from the airport at 12:00AM Tuesday.

The 24 clock is clear that 00:00 Tuesday is the beginning of Tuesday, or midnight on Monday. It is also clear that 24:00 Tuesday is midnight on Tuesday. However, the 12 hour clock format is ambiguous. 12:00AM Tuesday could be thought of as the middle of the night on Monday or Tuesday. For the sake of the friend waiting at the airport, it is important to get this right!

One way to avoid the problem would be for the friend to ask to be picked up at 12:01AM Tuesday, but this doesn’t work for a business computer system or business web site. We need an accurate answer to this.

Who uses the 12-hour clock format

The 12 hour clock time format using AM and PM is used in North America (United States, Canada and Mexico) , Australia and 14 other nations including Egypt, India, Ireland, New Zealand, Pakistan and Saudi Arabia. However, the 12 hour analogue clock is used all over the world, so really we all use the 12 hour clock time to some extent.

Why are there 24 hours in a day?

The prevailing thought is that it is the Egyptians that created the 12 hour clock time format. Egyptians used base 12 (duodecimal) for counting, they literally counted on their fingers, but not counting individual fingers, they counted the joints on the fingers (excluding thumbs). If you look at the palm of your hand and count between the creases in your fingers you’’ see there are 3 joints, and with 4 fingers you count to 12. With two parts of the day, morning and afternoon / evening, we have 24 hours in the day.


In answer to the questions posed:

  • What does AM PM stand for?
    – AM means Ante meridiem, after midnight
    – PM means Post meridiem, after midday
  • Is noon AM or PM?
    Technically noon is neither AM or PM, but the most sensible and most commonly used time for noon is 12:00PM.
  • What do businesses, web sites and software use?
    Most, but not all businesses, web sites, software and the most popular calendar software uses 12:00PM as noon.
  • When is 12:00AM?
    12:00AM is the start of the day, so midnight of the day before.
  • How does the 12 hour clock time format work?
    The 12 hour clock runs as follows:
    12:00AM, 1:00AM….11:00AM, 12:00PM, 1:00PM….11:00PM, 12:00AM the next day.
  • Who uses the 12 hour clock time format?
    There are 18 countries around the world that use the 12 hour clock.
  • Why are there 24 hours in a day?
    It is believed the Egyptians divided the day into two sets of 12 hours as they counted using base 12.

Why does all this matter to Timewatch?

Timewatch develop professional services automation software, including resource scheduling, calendar planning, time tracking, timesheet billing and add-ons for Outlook and Office 365. It is very important that time is precisely represented as any discrepancy is problematic. Over the years, many customers have asked about about the correct representation of AM and PM for noon and midnight as well as whether 12:00AM is at the end of the beginning of the day. This article is provided to all who wish to understand how AM / PM work and why.

Want to Learn More?

Timewatch has over 20 years experience developing add-ons for Outlook and helping customers broaden their use of Outlook. Even if you are interested in just one or two of the Outlook enhancements listed above Whitespace® can help. The system is configurable, so you can start with a simple configuration and add morefunctions as you need them.

If you have any questions or want to see the system in operation, contact us via chat, or book a free consultancy call with a product specialists.

Speak with a specialist

Learn more about Timewatch products such as our Outlook company calendar

Other posts you may be interested in:

AM PM explained. Is noon 12AM or 12PM? Why are there 24 hours in a day? (4)

More on AM PM from the Royal Observatory at Greenwich

These folks know a thing or two about time. They are world renowned experts in time and had a hand in inventing time for navigation.

AM PM explained. Is noon 12AM or 12PM? Why are there 24 hours in a day? (5)

12am & 12pm – What’s the difference?!

Another helpful article from The English Club. Interestingly in the comments many people still think the hour after noon is 12:XXAM.

AM PM explained. Is noon 12AM or 12PM? Why are there 24 hours in a day? (6)

Timewatch helps create Outlook group calendar – and gets AM-PM right!

Our multi resource calendar system for Outlook displays multiple employees calendars side by side over days, weeks or months, and yes, we display AM PM correctly.

AM PM explained. Is noon 12AM or 12PM? Why are there 24 hours in a day? (7)

Outlook has no analytics. You can’t query time by customer, project or employee. We solve that.

Does Outlook have any analytics? No, none. You can’t see employee calendars side by side for more than a couple of days, and even then there is not enough detail to be usedful.

Timewatch’s multi-employee add-on calendar for Outlook solves this. You can see as many people as you want over as many days as you want (within reason) and sort, search and filter by availability or capacity as well as access powerful analytical and reporting tools. It turns Outlook into a resource scheduling and company resource planning tool.

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Use calendar time in your timesheet

Outlook (and Google) appointments hold most if not all of the information needed for a timesheet, why not use this information to save staff time? With this Outlook add-on, employees’ calendar can be turned into timesheets, save employees hours each week and increasing the accuracy of timesheet.


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Why is there no 12pm? ›

There are no standards established for the meaning of 12am and 12pm. It is often said that 12am Monday is midnight on Monday morning and 12pm is midday. This puts all the times beginning with 12 and ending with am in the same one-hour block, similarly with those ending with pm.

Why is 24 hour time used instead of AM and PM and what do AM and PM even mean? ›

And then AM means Ante (before) Meridian ( mid day) and PM means Post (after) Meridian . Thus AM & PM terminogy is used for 12 hour cycle / frame to indicate if the time is before mid day or 12 Noon or after .. In 24 hour time frame AM & PM are superfluous and avoid extra digit in digital time display.

Why does AM and PM change at 12? ›

The 12-hour clock divides the 24-hour day into two periods. am stands for the Latin ante meridiem, translating to "before midday". This is the time before the sun has crossed the meridian. pm stands for post meridiem or "after midday" – after the sun has crossed the meridian.

Why does the day start at 12 am? ›

Since the highest point of the day was noon, the opposite has to be midnight that was when the 12 started over again, so that's why the day starts at midnight. The reason that the hands on a clock travel the way they do, or "clockwise," also goes back to the invention of the sundial in ancient Egypt.

Why is 12 PM not 12AM? ›

It's all because of the conventions used with the "12-hour clock." 12:00 am is usually taken to mean midnight (and 12:00 pm is noon). At 59 minutes past midnight it is therefore 12:59 am, but a minute later it's 1:00 am.

How did 12 PM become noon? ›

Why do we call 12 p.m. noon? The term 'noon' comes from the Old English 'non', which in turn is derived from the Latin 'nona hora' -ninth hour. Originally it referred to the ninth hour after sunrise - that is, 3pm. During the twelfth century, the meaning began to shift from 3pm to midday.

Why is AM PM so confusing? ›

"a.m." means "ante meridiem" which means "before midday" and p.m. means "post meridiem", which means "after midday". Consequently to use am/pm to refer to midday, (12noon) is simply wrong (and similarly midnight). The correct terms are 12 noon and 12 midnight, with times in between using am/pm.

Why do we use 24 hours in a day? ›

Our 24-hour day comes from the ancient Egyptians who divided day-time into 10 hours they measured with devices such as shadow clocks, and added a twilight hour at the beginning and another one at the end of the day-time, says Lomb. "Night-time was divided in 12 hours, based on the observations of stars.

Why do we still use 24-hour time? ›

The 24-hour clock is commonly used there only in some specialist areas (military, aviation, navigation, tourism, meteorology, astronomy, computing, logistics, emergency services, hospitals), where the ambiguities of the 12-hour notation are deemed too inconvenient, cumbersome, or dangerous.

Why is morning time called am? ›

Any time with a.m. after it should be regarded as the morning time. The term a.m. stands for the Latin phrase "ante meridiem." In English, this translates to "before midday." You can think of p.m. as the opposite of a.m. We associate p.m. with the afternoon and evening.

What does 5 past 10 mean? ›

When the time is 'five past', it is 5 minutes after the hour and the long hand (minute hand) points to the 1. When it is 'ten past', it is 10 minutes after the hour and the long hand (minute hand) points to the 2. For ten and five past, the short hand (hour hand) points just after the hour.

Why is time split into am and pm? ›

The first is referred to as am, which stands for ante meridiem, “before midday” in Latin, while the second is referred to using pm, meaning post meridiem, or “after midday.” This nomenclature is based on the position of the Sun in relation to the meridian.

Does the day start at 12 am or 12 01? ›

12:00AM is the start of the day, so midnight of the day before. How does the 12 hour clock time format work?

At what time does the day start? ›

Though there is no global unanimity on the issue, most often midnight is considered the start of a new day and is associated with the hour 00:00. Even in locales with this technical resolution, however, vernacular references to midnight as the end of any given day may be common.

Why is it called midnight? ›

It's no surprise that midnight is the very middle of the night, the word itself stemming from the Old English mid-niht, from mid, "among" or "in the middle of," and night, from niht, which appropriately means both "night" and "darkness."

Why is it 12 PM and not 0pm? ›

From the Latin words meridies (midday), ante (before) and post (after), the term ante meridiem (a.m.) means before midday and post meridiem (p.m.) means after midday. Since "noon" (midday, meridies (m.)) is neither before nor after itself, the terms a.m. and p.m. do not apply.

Why is 11pm later than 12 PM? ›

The exact middle of the day and night happens at 12:00 not 12:01, thus most of the minutes must belong to the period following. Because, with a 12-hour clock and a 24-hour day, you must change to “afternoon time” one hour after 11 in the morning; and back to “morning time” one hour after 11 in the evening.

Is the afternoon 12 PM or 12AM? ›

So it's in the first period which begins at midnight. So technically, 12AM and 12:30PM is just afternoon, which technically is 12PM. So to avoid confusion, many people do not use 12AM or 12PM, and prefer to use midnight for 12AM and noon for 12PM. So afternoon begins at noon, and runs until evening.

Is it 12 PM or am in the afternoon? ›

Answer: 12 O'clock in the afternoon is pm.

Ante meridiem is generally referred to as AM, am, a.m., or A.M. Whereas PM, pm, p.m., or P.M. are typically abbreviated post meridiem.


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