What Are AM and PM?
In many parts of the world, especially in the United States and other English-speaking countries, time is commonly expressed using the 12-hour clock format. This system uses the designations AM and PM to differentiate between the two halves of the day — but what exactly do these abbreviations mean?
The Origin of AM and PM
The abbreviations AM and PM come from Latin:
- AM = ante meridiem, meaning before noon
- PM = post meridiem, meaning after noon
In the 12-hour system:
- AM represents the time from midnight (12:00 AM) to noon (11:59 AM)
- PM covers the time from noon (12:00 PM) to midnight (11:59 PM)
Using numbers from 1 to 12 followed by “am” or “pm,” the system defines all 24 hours in a day.
Examples:
- 1:00 AM → one hour after midnight
- 5:00 AM → early morning
- 5:00 PM → late afternoon
- 11:00 PM → one hour before midnight
Midnight and Noon: AM or PM?
Here’s where it gets tricky: is midnight AM or PM? What about noon?
The Confusion
Logically, neither midnight nor noon is clearly before or after midday — making it hard to label them accurately as AM or PM.
Common Convention
Most sources and digital clocks follow this widely accepted convention:
- Midnight = 12:00 AM
- Noon = 12:00 PM
However, this can still lead to confusion.
Tip: To avoid ambiguity, it’s best to use the terms 12 noon and 12 midnight when scheduling important events.
Is Midnight the Start or End of a Day?
Another common issue arises when someone refers to 12:00 AM on a specific date.
Example:
If a flight is scheduled for 12:00 AM on April 13, do you show up right after April 12 ends, or at the end of April 13?
This ambiguity comes from the 12-hour system not attaching a clear date boundary to midnight.
Ways to Clarify:
- Use 12:01 AM for the start of a day.
- Use 11:59 PM to refer to the very end of a day.
- Alternatively, use the 24-hour clock format (military time), where:
- 0:00 = start of the day
- 24:00 = end of the day
12-Hour vs. 24-Hour Format
The 24-hour clock eliminates the need for AM and PM. Hours are counted from 0 to 24, starting at midnight.
| 12-Hour Time | 24-Hour Time |
|---|---|
| 12:00 AM (midnight) | 0:00 |
| 1:00 AM | 1:00 |
| 11:00 AM | 11:00 |
| 12:00 PM (noon) | 12:00 |
| 1:00 PM | 13:00 |
| 11:00 PM | 23:00 |
| 12:00 AM (midnight) | 24:00 (end of day) |
How to Convert Between 12-Hour and 24-Hour Time
Here’s a quick guide:
From 12-Hour to 24-Hour:
- 12:00 AM – 12:59 AM → Subtract 12 hours
- e.g., 12:49 AM → 0:49
- 1:00 AM – 11:59 AM → Keep the time
- e.g., 11:49 AM → 11:49
- 12:00 PM – 12:59 PM → Keep the time
- e.g., 12:49 PM → 12:49
- 1:00 PM – 11:59 PM → Add 12 hours
- e.g., 1:49 PM → 13:49
From 24-Hour to 12-Hour:
- 0:00 – 0:59 → Add 12 hours and AM
- e.g., 0:49 → 12:49 AM
- 1:00 – 11:59 → Add AM
- e.g., 11:49 → 11:49 AM
- 12:00 – 12:59 → Add PM
- e.g., 12:49 → 12:49 PM
- 13:00 – 23:59 → Subtract 12 and add PM
- e.g., 13:49 → 1:49 PM
Where Is the 12-Hour Format Used?
Most of the world uses the 24-hour clock, especially in formal settings and digital systems.
However, the 12-hour format with AM/PM is still the standard in these countries:
- United States
- Canada (except Québec)
- Australia
- New Zealand
- Philippines
Travelers visiting these regions should be familiar with AM and PM to avoid confusion.
A Brief History of Timekeeping
Why does the day have 24 hours, anyway?
This system dates back to Ancient Egypt, where the day was divided into:
- 12 hours of daylight
- 12 hours of night
They used sundials, shadow clocks, and water clocks, and tracked stars known as decans to measure night hours.
These ancient timekeeping methods laid the foundation for our modern 24-hour division of the day.
Final Thoughts
Understanding the difference between AM and PM — and how they’re used — is essential for clear communication, especially when scheduling events or working across time zones. The 12-hour clock is familiar and widely used, but it comes with a few quirks that are worth remembering:
- Use 12 noon or 12 midnight instead of relying on AM/PM
- Add the date when referencing midnight to avoid confusion
- Consider the 24-hour format for precision
Want to explore more about time, clocks, and time zones? Stick around at amofpm.com for more tips and tools on mastering your day — hour by hour.