12 AM or 12 PM? Noon, Midnight and How to Avoid Confusion
In everyday use 12 PM is noon and 12 AM is midnight at the start of the day. Why the labels are ambiguous, what NIST recommends, and how to write deadlines clearly.
12 PM is noon and 12 AM is midnight — specifically, the midnight at the start of a day. That is how phones, computers and digital clocks label them, and how most people read them. So 12:30 AM is half an hour after midnight, and 12:30 PM is half an hour after noon.
The labels still cause real confusion, which is why timekeeping authorities recommend avoiding them when it matters.
The sequence around noon and midnight
| 12-hour label | 24-hour time | What it is |
|---|---|---|
| 11:59 PM (Thursday) | 23:59 Thu | one minute before midnight |
| 12:00 AM (Friday) | 00:00 Fri | midnight, start of Friday |
| 12:01 AM (Friday) | 00:01 Fri | one minute after midnight |
| 11:59 AM | 11:59 | one minute before noon |
| 12:00 PM | 12:00 | noon |
| 12:01 PM | 12:01 | one minute after noon |
The pattern that trips people up is that the hour labelled “12” belongs to the following half of the day: 12 AM comes before 1 AM, and 12 PM comes before 1 PM. Counting “11 AM, 12 AM, 1 PM” feels natural but is wrong on a standard clock.
Why it is ambiguous in the first place
The abbreviations are Latin: a.m. is ante meridiem, “before midday”, and p.m. is post meridiem, “after midday”.
- Noon is the meridiem itself. It is neither before nor after noon, so strictly neither label applies.
- Midnight is both. It is twelve hours after one noon and twelve hours before the next.
The U.S. National Institute of Standards and Technology puts it bluntly in its Times of Day FAQ: “12 a.m. and 12 p.m. are ambiguous and should not be used.” NIST does accept the clock convention as reasonable, noting that the shortest measurable duration after noon should be designated p.m. — so a digital clock that switches to PM the moment it shows 12:00 is behaving sensibly.
Style guides have not always agreed. According to the history summarized in Wikipedia’s article on the 12-hour clock, the U.S. Government Printing Office style manual has switched conventions between editions: the 2000 edition gave 12 a.m. for noon and 12 p.m. for midnight, and the 2008 edition reversed that to the modern usage. An older convention wrote noon as “12 m.” (meridies). When an official style manual can flip, it is fair to say the labels are not self-explanatory.
What to write instead
The safest forms, roughly in order of preference for general writing:
- “Noon” and “midnight” as words. “The meeting starts at noon” is unambiguous. “12 noon” is also fine.
- 24-hour time. 12:00 is noon and 00:00 is midnight; there is no a.m./p.m. to misread. See 24-Hour Time and Military Time for conversions.
- A time one minute away from midnight, when the day matters: 11:59 PM or 12:01 AM.
“Midnight” on its own has a remaining problem: which day’s midnight. That is the source of most missed deadlines.
The “midnight on Friday” problem
Suppose a form says applications close “at midnight on Friday 30 October”. Two readings are both defensible:
- the midnight at the end of Friday (Friday night into Saturday), or
- the midnight at the start of Friday (Thursday night into Friday), since 12:00 AM Friday is 00:00 on Friday.
Most people mean the first, but a system configured to “Friday 00:00” will close a full day early. NIST addresses this directly: specifying an event as occurring on a particular day at 11:59 p.m. or 12:01 a.m. avoids ambiguity, “especially legal documents such as contracts and insurance policies.”
For deadlines, write:
- “23:59 on Friday 30 October” or “11:59 PM on Friday 30 October” — the last minute of Friday;
- or “by the end of Friday 30 October”, when a few seconds either way do not matter;
- and in every case, the time zone: “23:59 London time” or “23:59 UTC”. A deadline of “midnight” with no zone is 24 hours wide across the world. A time zone converter shows what a deadline means elsewhere.
For start times, 00:01 or 12:01 AM marks the beginning of a day without touching the boundary itself. Coverage periods and validity windows are often written this way, from 00:01 on the first day to 23:59 on the last.
If you set up a reminder for a deadline, a countdown to the exact instant — with the date and zone stated — removes the guesswork.
Midnight in ISO 8601 and computing
The international date and time standard, ISO 8601, writes midnight at the start of a day as 00:00. It also has a notation for midnight at the end of a day, 24:00: 2026-10-30T24:00 is the same instant as 2026-10-31T00:00. The 2019 revision of the standard dropped 24:00, and a 2022 amendment reinstated it for the end of a calendar day. NIST likewise suggests 0000 for midnight at the beginning of a day and 2400 for the end.
Software is less flexible. Most programming languages and databases only produce 00:00 and treat a day as running from 00:00:00 to 23:59:59.999…. When you store a deadline, store the instant (for example 2026-10-30T23:59:59+00:00, or 2026-10-31T00:00:00Z as an exclusive end) rather than the word “midnight”. The ISO 8601 guide covers the formats.
Quick reference
| You mean | Write | Avoid |
|---|---|---|
| Midday | noon, 12:00, 12 noon | 12 AM |
| Start of a day | 00:00 on 30 Oct, 12:01 AM on 30 Oct | ”midnight on 30 Oct” |
| End of a day | 23:59 on 30 Oct, end of 30 Oct | ”midnight on 30 Oct”, 12 AM 30 Oct |
| A deadline | 23:59 on Fri 30 Oct, UTC | ”midnight Friday” |
One last check worth doing: if a time ends in “12 AM” or “12 PM” and someone will act on it — a flight, a contract, a cron job — rewrite it in one of the forms in the left-hand column.
Sources
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