12-Hour vs 24-Hour Time: How to Convert and When to Use Each

Some countries write 3:00 pm, others 15:00. Here is how to convert between them quickly, and how to avoid the confusing cases at noon and midnight.

By the VClock team at Digimarket Group SIA · Updated 4 November 2026

The two systems

The 24-hour clock numbers hours from 00 to 23, starting at midnight. The 12-hour clock uses 1–12 twice a day with “am” (ante meridiem, before noon) and “pm” (post meridiem, after noon).

Converting

• 24 → 12: for hours 13–23, subtract 12 and add pm (17:30 → 5:30 pm). 12:xx is pm. 00:xx becomes 12:xx am. Hours 01–11 are am.
• 12 → 24: for pm times except 12 pm, add 12 (8:15 pm → 20:15). 12 am becomes 00. Am times stay the same.

Noon and midnight

“12 am” and “12 pm” confuse many people. To be unambiguous, write “noon” or “midnight”, or use 24-hour time. For deadlines, “23:59 on 30 November” is clearer than “midnight on 30 November”, which some readers take to mean the start of that day.

Who uses which

The 24-hour clock is standard in most of Europe, Asia, Latin America and Africa in writing, and in aviation, the military, medicine and computing worldwide. The 12-hour clock dominates everyday use in the United States, Canada (in English), Australia, the Philippines and some other countries, though many people there read both easily.

Tips for international communication

• Use 24-hour time in written invitations to international audiences.
• Add the time zone or city.
• Avoid “12 am/pm” for deadlines.