What Is a 24-Hour Clock?

The 24-hour clock numbers the hours of the day from 00 to 23, starting at midnight. It's the most common way of writing time around the world, and removes any doubt about morning and evening: 07:00 can only mean morning, and 19:00 can only mean evening.

Converting between the two

From the 24-hour clock to the 12-hour clock:

• 00:00 to 00:59: add 12 and use AM. So 00:30 is 12:30 AM.
• 01:00 to 11:59: the same numbers, with AM. So 09:15 is 9:15 AM.
• 12:00 to 12:59: the same numbers, with PM. So 12:45 is 12:45 PM.
• 13:00 to 23:59: subtract 12 and use PM. So 18:20 is 6:20 PM.

The confusion around midnight and noon

The 12-hour system is at its most confusing at these two moments. 12:00 PM means noon and 12:00 AM means midnight, though the abbreviations don't make that obvious. Which is why the 24-hour clock is preferred where mistakes are costly. In 24-hour time, midnight at the start of the day is 00:00 and noon is 12:00.

Where it's used

Most of Europe, Asia, Africa and Latin America use it in everyday writing, though people often speak in 12-hour terms. Aviation, the military, hospitals and public transport use it almost everywhere. The US, Canada, Australia and the Philippines tend to use the 12-hour clock day to day. In the US, the 24-hour clock is often simply called "military time".

Try it

The digital clock switches between both formats with one click, so you can see them side by side as you get used to them.