Learn when to use AM/PM versus 24-hour time, plus quick conversion tricks and common pitfalls.

This guide focuses on practical steps you can use immediately—whether you’re scheduling a call, planning travel, or publishing a time on a website.

What this article covers

You’ll learn a repeatable approach that works across US states, countries, and DST transitions. The goal is simple: never guess and never rely on memory for offsets.

Quick conversion rules

  • 12:00 AM = 00:00
  • 12:00 PM = 12:00
  • Add 12 for PM hours from 1–11 (1 PM → 13:00)
  • Keep AM hours as-is (9 AM → 09:00)

If you’re publishing schedules, 24-hour time reduces AM/PM mistakes, especially for international audiences.

The safest way to communicate time

When you publish time for other people:

  • Include the date (not just the time).
  • Include the time zone.
  • Use a 24-hour clock or AM/PM with a clear label.
  • If your audience spans regions, include a second reference zone.

When you schedule time for systems, store it as:

  • timestamp + IANA timezone (or timestamp in UTC).

Troubleshooting checklist

  • Is your device time zone set correctly?
  • Are you looking at the right date after conversion?
  • Does either location observe DST?
  • Are you mixing abbreviations (EST) with IANA zones (America/New_York)?
  • For recurring meetings: did DST change since the last meeting?

Frequently asked questions

  • Do I need to include the date as well as the time? Yes—especially when conversions cross midnight.
  • Is an abbreviation like “EST” always correct? Not always. Prefer ET or an IANA time zone name.
  • What’s the fastest way to confirm accuracy? Open a converter and verify with the time zone name, not just the offset.

Quick checklist you can use today

  • Write the date and time together.
  • Add a time zone label.
  • For international audiences, add a second zone or a converter.
  • Re-check around DST weeks.
  • For recurring events, let calendars handle conversion when possible.