10:30 o’clock

My sister-in-law will soon graduate from high school, and we recently got her graduation announcement in the mail. It was pretty standard stuff—a script font in metallic ink on nice paper—but one small detail caught my eye. It says the commencement exercises will take place at “ten-thirty o’clock.” As far as I can remember, I’ve never before heard a rule against using “o’clock” with times other than the hour, but it struck me as wrong.

I checked Merriam-Webster first, but it was no help; all it says is “according to the clock,” though its example sentence is “the time is three o’clock.” I then pulled out my copy of Merriam-Webster’s Dictionary of English Usage, but it didn’t even have an entry for o’clock or clock. So then, because my wife was on the computer and I couldn’t access the OED online, I pulled out my compact OED and magnifying glass to see if it had anything to say.

Once I had flipped to the entry and scanned through the minuscule type, I found this one line: “The hour of the day is expressed by a cardinal numeral, followed by a phrase which was originally of the clock, now only retained in formal phraseology; shortened subsequently to . . . o’clock.” The citations begin with Chaucer and continue up to modern English.

And then, out of curiosity, I checked the Corpus of Contemporary American English, but I couldn’t find any examples of x:30 o’clock. Google, however, turned up plenty of examples, including a thread on Amazon’s Askville asking why you can’t say “11:30 o’clock.” The best explanation there seems to be that since the clock hands aren’t pointing at a specific hour, it can’t be anything-o’clock.

This answer doesn’t seem quite satisfying to me—it doesn’t explain why the hour hand has to be pointing directly at a number or why the minute hand doesn’t matter. But then I remembered that clock originally meant “bell” and that early clocks chimed on the hour (well, I suppose some modern clocks do too, but you see where I’m going). Early mechanical clocks were rather large, and most people measured time not by checking the clock face to see where the hands were, but by counting the number of chimes on the hour. So I would assume that this is why it sounds strange to use “o’clock” with fractions of hours. Thoughts, anyone?