AM and PM Complete Guide
Updated June 17, 2026
AM stands for ante meridiem — Latin for "before midday." PM stands for post meridiem — Latin for "after midday." Together they divide the 24-hour day into two 12-hour periods. But there is more to it than the translation: when exactly does AM become PM? Is 12 AM midnight or noon? And should you write it as AM, a.m., or am? This guide covers all of it.
The short version
AM = before noon (midnight to 11:59 AM). PM = after noon (12:00 PM to 11:59 PM). 12 AM = midnight (00:00 in 24-hour time). 12 PM = noon (12:00 in 24-hour time). For writing: use AM/PM for digital, a.m./p.m. for print journalism, and stick to one style in any document. Use the converter to check any time.
What AM and PM actually mean
The terms come from Latin and refer to the sun's position relative to the meridian — an imaginary line running north-south across the sky that the sun crosses at solar noon.
AM, Ante Meridiem
Ante = before, meridiem = midday. Literally "before the sun crosses the meridian." Covers the hours from midnight through late morning.
PM, Post Meridiem
Post = after, meridiem = midday. Literally "after the sun crosses the meridian." Covers the hours from noon through late night.
Technically, exactly 12:00 noon is neither AM nor PM, the sun is directly on the meridian, meaning it is neither before nor after. But by convention, we call noon 12:00 PM. Some style guides acknowledge this technicality; most ignore it.
When each period starts and ends
12 AM vs 12 PM, the most common confusion
The 12-hour clock changes its AM/PM label at exactly 12:00, but the hour number does not reset to 1 until one hour later. This is why people mix up midnight and noon.
12:00 AM = Midnight
- Equals 00:00 in 24-hour time
- Marks the start of a new day
- 12:30 AM is still after midnight
- In military time: spoken as "zero hundred hours"
12:00 PM = Noon
- Equals 12:00 in 24-hour time
- Marks the middle of the day
- 12:30 PM is after noon
- In military time: spoken as "twelve hundred hours"
To avoid all ambiguity: write "midnight" or "noon" in words, or use 24-hour time. A deadline of "midnight Friday" is clearer than "12:00 AM Friday." When you must use 12-hour time with AM/PM, add the word for clarity: "12:00 AM (midnight)."
How to write AM and PM, style guide comparison
There is no single correct format. Which one you use depends on what you are writing and who will read it.
| Format | Example | Recommended by | Best for |
|---|---|---|---|
| a.m. / p.m. | 9:00 a.m., 6:00 p.m. | AP Stylebook, Chicago Manual | Newspapers, books, academic writing |
| AM / PM | 9:00 AM, 6:00 PM | Microsoft Manual, US Government | Websites, apps, technical docs |
| am / pm | 9am, 6pm | The Guardian, BBC, UK style | British publications, casual writing |
| A.M. / P.M. | 9:00 A.M. | Some older academic styles | Rare, lowercase is more common today |
Rule of thumb: Use AM/PM for anything online. Use a.m./p.m. for formal print writing. Always put a space between the time and the label — "9 AM" not "9AM." And never mix styles in the same document.
Common AM/PM mistakes
Writing "12 AM" when you mean "12 PM" (or vice versa). This is the single most common time-format error. If your calendar event, medication schedule, or flight booking uses the wrong one, you are off by 12 hours. When in doubt, write "midnight" or "noon" in words.
Forgetting the space. "9AM" without a space is never correct in formal writing. Always include a space or a non-breaking space: "9 AM" or "9 AM."
Mixing AM/PM with 24-hour time. "16:00 PM" is wrong — 16:00 already means 4 PM in 24-hour time. Adding PM is redundant. If you use 24-hour format, drop the AM/PM label entirely.
Assuming AM means "after midnight." AM does not stand for "after midnight." It stands for ante meridiem — before midday. 10:00 AM is not after midnight; it is before noon. The distinction matters because PM also covers hours after midnight, just a different midnight.