AM stands for ante meridiem — Latin for “before midday.” PM stands for post meridiem — Latin for “after midday.” The dividing line is noon, when the sun crosses the meridian. That’s the short answer. Below: the Latin origin, the awkward 12 AM / 12 PM rule that catches everyone out, and how to switch between 12-hour and 24-hour time.
What AM and PM mean
| Abbreviation | Latin | English | Hours covered |
|---|---|---|---|
| AM | ante meridiem | before midday | 12:00 AM (midnight) → 11:59 AM |
| PM | post meridiem | after midday | 12:00 PM (noon) → 11:59 PM |
Meridiem comes from the Latin meridies, meaning “midday.” It is also the root of “meridian,” the imaginary line the sun crosses overhead at noon.
The 12 AM and 12 PM problem
Midnight and noon are the two awkward points on a 12-hour clock — neither is strictly “before” or “after” itself. Convention solved it like this:
- 12:00 AM = midnight (the start of a new day)
- 12:00 PM = noon (halfway through the day)
To avoid the confusion, U.S. style guides such as the AP Stylebook recommend writing simply “midnight” and “noon” rather than 12 AM or 12 PM.
How to write AM and PM correctly
All four of these are accepted in writing: AM, am, A.M., a.m. Pick one style and stay consistent. The U.S. AP Stylebook uses lowercase with periods (a.m. / p.m.). The Chicago Manual of Style allows small capitals or lowercase with periods. Most digital products (Apple, Google, Microsoft) use uppercase without periods (AM / PM).
Place a single space between the time and the abbreviation: 10:30 AM, not 10:30AM.
Converting 12-hour to 24-hour time
The 24-hour clock — also called military time in the U.S. — counts hours from 0 to 23 starting at midnight. No AM/PM needed.
- 12:00 AM – 12:59 AM → subtract 12.
12:49 AM = 00:49 - 1:00 AM – 11:59 AM → leave it.
11:49 AM = 11:49 - 12:00 PM – 12:59 PM → leave it.
12:49 PM = 12:49 - 1:00 PM – 11:59 PM → add 12.
1:49 PM = 13:49
Converting 24-hour to 12-hour time
- 00:00 – 00:59 → add 12, label AM.
00:49 = 12:49 AM - 01:00 – 11:59 → add AM.
11:49 = 11:49 AM - 12:00 – 12:59 → add PM.
12:49 = 12:49 PM - 13:00 – 23:59 → subtract 12, label PM.
13:49 = 1:49 PM
Where each format is used
- 12-hour clock with AM/PM — United States, Canada (informal), Australia, the U.K., the Philippines, India
- 24-hour clock — Most of Europe, Latin America, the U.S. military, aviation, hospitals, public transport schedules worldwide
A short history
The 12-hour split goes back to ancient Egypt, where daytime and night-time were each divided into 12 unequal hours that stretched and shrank with the seasons. The Latin ante meridiem / post meridiem labels were standardized through the Middle Ages and adopted by mechanical clocks from roughly the 14th century onward. The 24-hour clock spread later through railroads, the military, and international aviation.
FAQs
What does AM stand for?
Ante meridiem — Latin for “before midday.”
What does PM stand for?
Post meridiem — Latin for “after midday.”
Is 12 AM noon or midnight?
12 AM is midnight. 12 PM is noon. Many style guides recommend writing “midnight” and “noon” instead to remove the ambiguity.
Is “12 noon” or “12:00 noon” correct?
Either works in casual writing. In formal writing, just “noon” or “midnight” by itself is cleanest.
What is military time?
Military time is the U.S. name for the 24-hour clock, written without a colon (e.g., 1349 = 1:49 PM). Used by the military, aviation, and many hospitals.
Why does AM start at midnight if midnight isn’t really “before” itself?
Because the day rolls over at midnight. Once the clock crosses 12:00 AM, every hour up to 11:59 AM is technically before the next noon — so it is “ante meridiem.”
Should I write am, AM, or A.M.?
All accepted. Pick one and use it consistently. AP style is lowercase with periods (a.m.). Digital products tend to use uppercase without periods (AM).
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