Updated May 19, 2026

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Many of us write or say “12 p.m.” (or “12:00 p.m.”) when we mean noon and “12 a.m.” when we mean midnight. This seems reasonable enough, at least intuitively. After all, the increments of time that follow noon are p.m.; those that follow midnight are a.m.

But “p.m.” and “a.m.” have specific meanings that don’t support either of those forms. The abbreviation “m.” stands for meridies, the Latin word for noon; “p.” stands for post (after); and “a.” stands for ante (before). So “p.m.,” or post meridiem (Latin nouns are inflected), means “after noon,” and “a.m.,” or ante meridiem, means “before noon.”

Exactly twelve o’clock in the middle of the day is noon—it can’t be after noon also—so “12 p.m.” isn’t logical.* There’s maybe a better case for “12 a.m.” as midnight, because midnight is technically before the next noon. But it’s also after the last one.

Confusing? Yes, which is why it’s usually best to use words for these two different times instead of numbers: The terms noon and midnight are always unambiguous.†

What about the twenty-four-hour system? That’s a little clearer—12:00 is always noon. But for midnight, there are two choices: 24:00 (for midnight at the end of one day) and 00:00 (for midnight at the start of the next).

And what about your phone? On most computers and other devices set to use the twelve-hour system, 12 p.m. is noon and 12 a.m. is midnight, a convenience that glosses over the precise meanings of those abbreviations.

For another look at this issue, see this explanation from the Physical Measurement Laboratory of the National (US) Institute of Standards and Technology, or this one from the UK’s National Physical Laboratory.

Until next time.

* The form “12 m.” might work for noon, but there are problems with that expression. For starters, “m.” is likely to be mistaken for “midnight.”

† Or almost always. The exact date for any given midnight may not be clear unless you specify a two-day range, as in “midnight, May 19–20.”

Photo: Big Ben at noon, by StefanW (via Wikimedia Commons).

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