What is am and pm
Last updated: April 1, 2026
Key Facts
- AM stands for 'Ante Meridiem,' Latin for 'before noon,' while PM stands for 'Post Meridiem,' meaning 'after noon'
- The 12-hour clock system divides each day into two 12-hour periods starting at 12:00 AM (midnight)
- Noon is 12:00 PM and midnight is 12:00 AM, representing the transition points between AM and PM
- Most English-speaking countries use the 12-hour format with AM/PM notation, while many others use 24-hour time
- AM/PM notation has been used for centuries, originating from Latin terminology in astronomical and ecclesiastical time-keeping
Understanding AM and PM
The 12-hour clock system with AM (Ante Meridiem) and PM (Post Meridiem) is one of the most common ways to express time in everyday life. These Latin abbreviations help distinguish between times in the first half of the day and the second half, providing clarity when discussing specific times without mentioning the full date.
AM: Before Noon
AM covers the time period from 12:00 AM (midnight) through 11:59 AM. The day begins at midnight (12:00 AM) and progresses through the early morning hours, morning hours, and into the late morning hours approaching noon. The AM period encompasses six hours of darkness followed by several hours of daylight in most temperate locations.
- 12:00 AM - 12:59 AM: Midnight hour
- 1:00 AM - 4:59 AM: Early morning
- 5:00 AM - 8:59 AM: Morning
- 9:00 AM - 11:59 AM: Late morning
PM: After Noon
PM covers the time period from 12:00 PM (noon) through 11:59 PM. Noon marks the transition from AM to PM and represents the midpoint of the day. The afternoon and evening hours continue until 11:59 PM, when the clock resets to 12:00 AM (midnight).
- 12:00 PM - 12:59 PM: Noon hour
- 1:00 PM - 4:59 PM: Afternoon
- 5:00 PM - 8:59 PM: Evening
- 9:00 PM - 11:59 PM: Night
Common Confusion Points
A frequent source of confusion is the transition at noon and midnight. 12:00 AM is midnight, marking the very beginning of the day, not the end. Similarly, 12:00 PM is noon, the middle of the day. After 11:59 AM comes 12:00 PM (noon), and after 11:59 PM comes 12:00 AM (midnight).
12-Hour vs. 24-Hour Time
The 12-hour format with AM/PM notation is primarily used in the United States, Canada, the United Kingdom, and several other English-speaking countries. Many other countries and most military, medical, and scientific contexts use 24-hour time, which numbers hours from 0-23, eliminating the need for AM/PM designations.
Historical Origins
The AM/PM system originates from Latin astronomical and ecclesiastical traditions. The use of 12-hour cycles dates back to ancient civilizations' observations of the sun's path. The formalization of this system became standard with the invention of mechanical clocks in medieval Europe.
Related Questions
What time is 12:00 AM vs 12:00 PM?
12:00 AM is midnight, the start of the day. 12:00 PM is noon, the middle of the day. This transition is confusing because the hour numbering resets to 12 at both midnight and noon instead of 0 and 12.
How do you convert 12-hour time to 24-hour time?
For AM times, keep the hour but drop AM (except midnight becomes 00:00). For PM times, add 12 to the hour (except noon remains 12:00). For example: 3:00 PM becomes 15:00, and 3:00 AM becomes 03:00.
Why do we use AM and PM instead of 24-hour time?
The 12-hour format is traditional in many English-speaking countries and remains common in daily life due to habit and tradition. It aligns with natural light cycles, making it intuitive for everyday scheduling.
Sources
- Wikipedia - 12-hour Clock CC-BY-SA-4.0
- Wikipedia - Hour CC-BY-SA-4.0