The 24-hour clock format, also known as military time, eliminates the need for AM/PM. Hours from 00 to 11 are AM, and hours from 12 to 23 are PM. To convert a PM time, simply add 12 to the hour (e.g., 3 PM = 15:00). To convert from 24-hour format, subtract 12 from hours greater than 12 (e.g., 17:00 = 5 PM).
Why the 24-hour clock exists
The 12-hour AM/PM system is used in everyday conversation in the US, Canada, Australia, and a few other countries. But the 24-hour format is the standard in most of the world, especially in military, aviation, healthcare, computing, and transportation contexts. The 24-hour format eliminates ambiguity: 3:00 AM and 3:00 PM are clearly different times (03:00 vs 15:00), whereas "3 o'clock" in the 12-hour format requires context to determine which one.
Quick conversion reference
- 1:00 AM = 01:00
- 6:00 AM = 06:00
- 12:00 PM (noon) = 12:00
- 1:00 PM = 13:00
- 6:00 PM = 18:00
- 12:00 AM (midnight) = 00:00
- 11:59 PM = 23:59
Where the 24-hour clock is used
Most of continental Europe, Latin America, Asia, and Africa use the 24-hour clock in everyday life. In France, "15h" means 3 PM. In Germany, "15 Uhr" means 3 PM. Military forces worldwide use 24-hour time exclusively (which is why it's called "military time" in the US). Airlines, train schedules, and bus timetables worldwide use 24-hour time to prevent confusion. Computing systems (Unix timestamps, ISO 8601 dates, cron jobs) all use 24-hour time internally.