24-Hour vs 12-Hour Clock: Conversion Chart, 12 AM vs 12 PM and Military Time

To convert 24-hour time to 12-hour time, subtract 12 from any hour above 12 and add PM: 17:45 is 5:45 PM. Hours 01 to 11 keep their number and take AM. The two tricky hours are 00, which becomes 12 AM, and 12, which stays 12 and takes PM. The sections below give the full chart and untangle noon and midnight.

How to convert between 24-hour and 12-hour time#

Only the hour and the AM/PM label change. Minutes and seconds stay as they are.

To go the other way, from 12-hour to 24-hour:

1. 1:00 to 11:59 AM: keep the hour and add a leading zero if it has one digit. 9:30 AM is 09:30.
2. 12:00 to 12:59 AM: replace 12 with 00. 12:30 AM is 00:30.
3. 12:00 to 12:59 PM: keep 12. 12:30 PM is 12:30.
4. 1:00 to 11:59 PM: add 12. 3:05 PM is 15:05, and 11:59 PM is 23:59.

The full conversion chart#

Military format is the same digits without the colon, so one chart covers both.

The labels "midnight" and "noon" in the first row follow the most common convention, which is not a rule, as the next section explains.

Why 12 AM and 12 PM confuse people, and what to write instead#

AM and PM abbreviate the Latin ante meridiem ("before midday") and post meridiem ("after midday"). Noon is midday itself, so it is neither before nor after it. Midnight sits twelve hours before noon and twelve hours after it (NIST). Strictly, neither label fits either moment.

Most people fill the gap with a convention: 12 AM is midnight and 12 PM is noon. The UK's National Physical Laboratory calls this generally accepted, but says no official standard sets it and that some people read 12 AM as the start of the day and 12 PM as its end (NPL). The software we checked follows the common reading: Node.js 22 formatting for US English prints midnight as "12:00 AM" and noon as "12:00 PM", and so does Timelive.

The official advice is to avoid the labels. NIST says 12 a.m. and 12 p.m. should not be used. NPL says to avoid them and suggests the 24-hour clock or the words noon, midday and midnight. The Australian Government Style Manual says to use "noon", "midday" or "midnight" instead, because it is easier to understand (Style Manual).

Midnight has a second problem: which day does it belong to? NIST suggests naming the day together with a time such as 11:59 p.m. or 12:01 a.m., which matters most in contracts and insurance policies. Where the exact time has legal force, follow the wording of the contract or the official source. For ordinary use, these are clearer ways to write the same moments (October 16, 2026 is a Friday):

Military time: the 24-hour clock written as hhmm#

Military time is the 24-hour clock plus a few conventions for writing and saying it (timeanddate). You write four digits with no colon, and the hour always has two digits (0100). You say the leading zero, and you say "hundred" for a full hour: 1000 is "ten hundred", never "one thousand". For times between full hours you drop "hundred".

Ordinary 24-hour time keeps the colon (16:15), as the Language Portal of Canada writes it, with a colon before the minutes and no a.m. or p.m. Military time adds the no-colon style, the "hundred" habit and sometimes a zone letter after the digits. The letter Z, spoken "Zulu", means UTC, so 1400Z is 14:00 UTC (UTC vs GMT: What Is the Difference and Which Should You Use?). Guides differ on small points, such as whether the leading zero is read "zero" or "oh" and whether "hours" follows every time.

Midnight: 00:00 or 24:00?#

Both name the same instant, seen from two sides. 24:00 on Friday, October 16, 2026 is the end of that day, and 00:00 on Saturday, October 17 is the start of the next. NIST describes 0000 as midnight at the beginning of a given day and 2400 as the end of a given day. The Language Portal of Canada says midnight may be written either way, with 24:00:00 ending one day and 00:00:00 beginning the next, depending on context (Language Portal of Canada).

ISO 8601, the international standard for numeric dates and times, has changed on this point. As of October 2026, the committee's page for the 2022 amendment to ISO 8601-1 lists the re-introduction of "ending of day" among its aims, which implies the 2019 edition had dropped it (ISO/TC 154). The standard's text is paywalled, so check it if strict conformance matters.

Software does not agree either. In Node.js 22.22.0, new Date("2026-10-16T24:00:00Z") is accepted and gives 2026-10-17T00:00:00.000Z. In Python 3.13, datetime.fromisoformat("2026-10-16T24:00:00") raises an error ("hour must be in 0..23"). For stored data, use 00:00 of the following day; Unix Time, ISO 8601 and UTC: How Computers Store Time covers how computers store time. In writing, use 00:00 for when something starts and keep 24:00 for the end of a range, where the format allows it.

Which countries use which clock#

Writing, speaking and software often differ within one country, so any list is a simplification. Here is what the references say:

• Worldwide: the 24-hour clock is the most widely used format (timeanddate), and Canada's Language Portal calls it the most commonly used system internationally.
• Spoken 12-hour time: Wikipedia's 12-hour clock article lists the United Kingdom, Ireland, the United States, Canada outside Quebec, Australia, New Zealand, South Africa, India, Pakistan and Bangladesh, plus Mexico, Colombia and the Philippines, as places where it is the main system. It adds that some countries in Europe and Latin America speak in 12-hour form but write in 24-hour form (Wikipedia). This is a secondary source.
• Official guidance: the Language Portal of Canada recommends the 24-hour system for documents in both official languages and for international communications. The Australian Government Style Manual uses the 12-hour clock for most text and the 24-hour clock for content such as timetables and schedules, or where space is tight.

Phones and browsers pick a default from your region. This is how the locale data in Node.js 22.22.0 (ICU 77.1, CLDR 47) shows 15:30, computed in this session:

These are software defaults, not a census of habits. The British English default is 24-hour, for example, while Wikipedia's list above counts the UK among the 12-hour places.

To read a 24-hour time in another city, enter it in the time zone converter. For the story of why a day has 24 hours, see How Ancient People Told Time: Shadows, Sundials and Water Clocks.

Frequently asked questions

Sources and further reading

1. Times of Day FAQs — NIST
2. Midnight - is it 12am or 12pm? — National Physical Laboratory
3. Quick guide: dates and time — Australian Government Style Manual
4. Time of day — Language Portal of Canada
5. ISO 8601-1:2019/Amd 1:2022 — ISO/TC 154
6. Military Time: The 24 Hour Clock — timeanddate.com
7. 12-hour clock — Wikipedia