Midnight and Noon—When to Use A.M. and P.M.

Here’s the short answer—12 p.m. usually refers to noon in the 12-hour clock system. If you’re talking about midnight, you’d want to say 12:00 a.m.

Generally, we use the 12-hour clock in the United States. That means you can count twelve hours from midnight through midday. Those twelve hours repeat again during a 24-hour cycle. Looking at a digital clock, you can see that the switch to p.m. happen at noon. 

In the context of military time, you’ll often see the 24-hour clock used. When someone talks about 1500 hours, they’re referring to the 24-hour clock. 1500 translates to 3 p.m. It’s three hours past noon, and twelve (noon) plus three equals fifteen. When you’re using the 24-hour clock, you eliminate any confusion about 12 a.m. and 12 p.m. Noon is 1200; midnight is either 0000 or 2400.

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What Do the Abbreviations A.M. and P.M. Mean?

The abbreviations a.m. and p.m. come from the Latin terms ante meridiem and post meridiem. In English, ante meridiem means before midday; post meridiem refers to the time after midday. Think of it this way—12:00 a.m. marks the beginning of the day. After noon, the count resets for the second half of the day. Technically, noon does not occur after midday—it happens precisely in the middle of the day. For this reason, you may cause confusion, even when you use 12 a.m. and 12 p.m. correctly. 

To avoid misunderstandings, it’s best to use noon or midnight to refer to 12:00 p.m. and 12:00 a.m. If you plan to meet someone at 12:00 a.m. on a particular date, use caution. Your date may end up meeting you at the wrong time or even on the wrong day! Sometimes, people assume that midnight is still part of the previous day, even though most argue that midnight marks the start of the new day. As an example, the few seconds after midnight may belong to Monday morning, while the moments before midnight belong to Sunday night. During New Years celebrations, the stroke of midnight marks the beginning of the new year. 

Where Did the 12-Hour System Originate?

According to Vermont Public Radio, “The reason a new day starts at 12:00 goes back to ancient Egypt when the day was measured using sundials.” As you can imagine, the sundial showed no shadow at midday; whereas, the dial showed a shadow throughout the remainder of the hours from sunrise to sunset. From sundown to sunrise, the sundial also provided no information. To account for the period of time without light, people repeated the same count as during the daylight hours. Midnight served as the counterpoint to noon. Since Egypt is in the northern hemisphere, the shadows cast by the sun’s movement from east to west made the sundial’s shadow move clockwise. That’s where the clockwise movement of modern clocks originated. 

Check Your Favorite Style Guide

When you’re writing a.m. and p.m., it’s important to keep in mind that different style guides handle the abbreviations differently. Here are a few examples:

MLA Style | a.m. and p.m.

Chicago Style | a.m. and p.m. or am and pm

APA Style | a.m. and p.m.

A Few Time-Zone Abbreviations

If you want to get familiar with other time-related abbreviations, this list is a great place to start. 

GMT | Greenwich Mean Time

UCT |  Universal Coordinated Time

ECT | European Central Time

EET | Eastern European Time

ART | Egypt Standard Time

EAT | Eastern African Time

MET | Middle East Time

NET | Near East Time

PLT | Pakistan Lahore Time

IST | India Standard Time

BST | Bangladesh Standard Time

VST | Vietnam Standard Time

CTT | China Taiwan Time

JST | Japan Standard Time

ACT | Australia Central Time

AET | Australia Eastern Time

SST | Solomon Standard Time

NST | New Zealand Standard Time

MIT | Midway Islands Time

HST | Hawaii Standard Time

AST | Alaska Standard Time

PST | Pacific Standard Time

PNT | Phoenix Standard Time

MST | Mountain Standard Time 

CST | Central Standard Time

EST | Eastern Standard Time

IET | Indiana Eastern Standard Time

PRT | Puerto Rico and US Virgin Islands Time

CNT | Canada Newfoundland Time

AGT | Argentina Standard Time

BET | Brazil Eastern Time

CAT | Central African Time

Sources:

  1. https://www.vpr.org/post/why-do-days-start-12-oclock
  2. https://24timezones.com/time-zones
  3. https://www.mlanet.org/page/mla-style-manual-numbers-and-dates
  4. https://writingexplained.org/chicago-style/am-pm