How to Read and Use the 24-Hour Clock

What is 00:00 in 12-hour time?

00:00 is midnight. It is the start of the day, not the end of one.

A single day has 24 hours. Number them 0 through 23 and every hour gets a unique label. No AM. No PM. No ambiguity about whether a time falls in the morning or the evening. One hour after midnight is 01:00. Noon lands at 12:00. One hour past noon is 13:00. The day ends at 23:59, then resets.

This format is the international standard (ISO 8601). Healthcare, aviation, railways and computing all rely on it. In the United States it is often called "military time."

How to convert a 12-hour clock to 24-hour

The conversion depends on whether the time is before or after noon.

Morning times (12:00 AM to 11:59 AM):

Keep the same hour, adding a leading zero where needed. 1:00 AM is 01:00. 9:30 AM is 09:30. The single exception: 12:00 AM turns into 00:00.

Afternoon and evening times (12:00 PM to 11:59 PM):

Add 12 to the hour. 1:00 PM is 13:00. 6:30 PM is 18:30. 11:00 PM is 23:00.

Table: 12-hour to 24-hour conversion

Minutes stay the same. 2:45 PM is 14:45. 10:15 AM is 10:15.

How to convert a 24-hour clock to 12-hour

Reverse the logic.

Hours 00:00 to 11:59: The time is AM. 00:00 is 12:00 AM. 01:00 is 1:00 AM. 11:30 is 11:30 AM.

Hours 12:00 to 23:59: The time is PM. Subtract 12 from the hour. 13:00 is 1:00 PM. 18:30 is 6:30 PM. 23:00 is 11:00 PM.

Quick rule: If the hour is 13 or higher, subtract 12 and add "PM". If the hour is 00, it is 12 AM.

Is midnight 00:00 or 24:00?

Midnight is 00:00. That marks the start of the day.

You will occasionally see 24:00 used to mean the end of a day. A business might list its hours as "07:00-24:00" to say it closes at midnight. But ISO 8601 uses 00:00 for midnight. Noon is always 12:00.

This is not a trivial distinction. A medication schedule that says "take at 00:00" means take at the start of the day. One that says "take at 24:00" is ambiguous and should be avoided.

Countries that use the 24-hour system

Most of the world runs on it. The United Kingdom, France, Germany, Spain, Italy, Australia and Japan all treat the 24-hour clock as standard. Train schedules, television listings and digital clocks show it by default.

The United States, India and the Philippines are exceptions. Daily life in those places defaults to the 12-hour clock. Even so, the US military, aviation, healthcare and emergency services all use the 24-hour format.

24-hour time in healthcare and aviation

Two industries where an AM/PM mistake can kill.

Healthcare: Medication timing uses the 24-hour format. A schedule for "q4h" (every 4 hours) might list doses at 06:00, 10:00, 14:00, 18:00, 22:00 and 02:00. A nurse reading "2:00 AM" instead of "2:00 PM" could give a dose 12 hours late. Shift handovers and surgery duration logs also stick to 24-hour time.

Aviation: Flight plans, weather reports and air traffic control communications use a 24-hour clock referenced to Coordinated Universal Time (UTC). A flight departing at 14:30 Z (Zulu time, another name for UTC) is unambiguous. Pilots and controllers never wonder if a departure is morning or evening.

How do I switch between 12-hour and 24-hour formats?

A time format converter handles the swap instantly. Enter a time in one format, get the equivalent in the other. Useful for payroll, project scheduling or logging work hours. The Time Format Converter works with 12-hour, 24-hour and decimal hours for payroll.

Anyone who schedules international meetings, manages a team across time zones or tracks medication timing should default to the 24-hour clock. The 12-hour clock is fine for casual conversation. For anything where a misread matters, use 00:00 to 23:59.