Is 12 PM noon or midnight?

Why 12 AM and 12 PM confuse people

AM and PM are abbreviations of the Latin ante meridiem and post meridiem, “before midday” and “after midday”. Every hour fits that rule except the two that start at 12. Noon is neither before nor after noon, and midnight is both twelve hours before and twelve hours after it. The US National Institute of Standards and Technology (NIST) therefore says 12 a.m. and 12 p.m. are ambiguous and should not be used.

The convention everyone follows anyway: the moment the clock reads 12:00 at midday it is PM, and at midnight it is AM. NIST agrees that the shortest measurable time after noon is PM, so a digital clock should switch to PM the instant 12:00 appears at midday, not a minute later. That is why 11:59 AM is followed by 12:00 PM, and 11:59 PM by 12:00 AM.

What to write instead

• Noon and midnight in words: a shop can say “open 8 a.m. to noon” or “3:30 p.m. until midnight”, both NIST’s own examples.
• One minute off for deadlines: NIST suggests 11:59 p.m. or 12:01 a.m. on a stated date, especially in contracts and insurance policies.
• The 24-hour clock: NIST’s other option is 0000 for midnight at the start of a date and 2400 for its end. 1200 can only be noon.

“Midnight on Friday”

NIST’s example: “midnight tonight” is clear, but “midnight on Friday, October 20th” is not: it can mean the start of Friday (the night between Thursday and Friday) or its end (the night between Friday and Saturday). A concert “at 12 AM Saturday” starts as Friday night ends. A form due “by midnight Friday” is usually meant as the end of Friday, but write 11:59 PM Friday and nobody has to guess.

12:30 AM vs 12:30 PM

The same rule covers the whole 12 o’clock hour: 12:30 AM is half an hour after midnight (00:30), and 12:30 PM is half an hour after noon (12:30). Converting to 24-hour time: for AM times, 12 becomes 00; for PM times from 1 PM on, add 12 (3 PM is 15:00); 12 PM stays 12.