24-Hour Clock vs 12-Hour Clock
The 24-hour clock counts the day from 00:00 to 23:59. The 12-hour clock splits the day into two 12-hour periods and uses AM or PM to show whether the time is before or after noon.
Direct answer
Use the 24-hour clock when you need unambiguous time. Use the 12-hour clock when your audience expects AM/PM time. For example, 18:00 in 24-hour time is 6:00 PM in 12-hour time.
Main differences
The 24-hour clock has no AM or PM. Each hour has one value, so 06:00 means morning and 18:00 means evening. This format is common in transportation, health care, the military, scheduling systems, and many countries outside the United States.
The 12-hour clock repeats the numbers 1 through 12 twice. AM covers midnight through the morning. PM covers noon through the evening. Because the same hour number appears twice each day, AM and PM are required for clarity.
Quick examples
How to convert
If the 24-hour time is 00:00, write 12:00 AM. If the hour is 01 through 11, keep the hour and add AM. If the hour is 12, keep 12 and add PM. If the hour is 13 through 23, subtract 12 and add PM.