CMOS 9.38 in the Spotlight
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 a.m. and p.m. are very specific: “m.” means meridies, the Latin word for noon. So “p.m.,” or post meridiem (Latin nouns take different endings depending on case), means “after noon.” Exactly twelve o’clock in the middle of the day is noon; it can’t be after noon also. There’s maybe a better case for 12 a.m. as midnight, because “a.m.” stands for ante meridiem (ante means “before”), and midnight is before the next noon. But it’s also after the last one.
Confusing? Yes, it is. (Apparently some of you who took last week’s quiz thought so too. Question no. 9 was tricky!) That’s why it’s best to use words instead of numbers: the terms “noon” and “midnight” are always unambiguous. For examples, including how to specify a two-day range with midnight, see CMOS 9.38.
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).
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.
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