Why You Have Probably Been Using 12 Am and 12 Pm Completely Wrong

An insightful review of Why You Have Probably Been Using 12 Am and 12 Pm Completely Wrong—check out what experts are saying.

Because 12:00 sits precisely on the threshold, applying directional prefixes introduces immediate operational friction. The National Institute of Standards and Technology (NIST) explicitly addresses this dilemma in its official timekeeping advisories. NIST states that terms like "12:00 AM" and "12:00 PM" are ambiguous and should never be used in legal documents, technical specifications, or official schedules.

The UK National Physical Laboratory (NPL) echoes this guidance. Its time-frequency division points out that noon is neither ante nor post meridiem. If an organization designates noon as 12:00 PM, logic would dictate that midnight should be 12:00 AM. But which day does that midnight belong to?

Midnight ambiguity creates systemic headaches. A calendar day begins at 00:00 and concludes at 24:00. If an insurance policy states coverage terminates at "12:00 AM on July 10," does protection expire as the sun rises on July 10, or does it persist until the final second of July 10 fades into July 11? Courts in both the United States and the United Kingdom have adjudicated bitter multi-million-dollar disputes born solely from this grammatical blind spot.