Why Does ‘a Quarter of’ Mean the Same Thing as ‘a Quarter to’?

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What does ”a quarter of 10” mean to you? A friend asked me this question recently and I thought the answer was obvious: It means 15 minutes before 10 o’clock. But there are apparently many people who find this construction confusing or have never heard of it. Why do some of us say “a quarter of” instead of “a quarter to” or “a quarter before”?

My first thought was that this “of” must be a holdover from some old construction that has fallen out of use, perhaps related to phrases like “of the clock” which eventually was shortened to “o’clock.” But according to the Oxford English Dictionary, the “quarter of” type of time-telling dates to 1817, which is not all that long ago in language history terms (“of the clock” dates all the way back to Chaucer). The first citation (“At 15 minutes of 10 a.m….”) is from an Indiana journal, and the construction appears to be chiefly American, though it is also found in Scottish and Irish English. 

It may have originated with the phrase type “It wants/lacks a quarter of 10.” This way of telling time goes way back and was used in both England and the U.S. Here, from the Antiquarian Horological Society, is an example of someone complaining about the inaccuracy of public clocks in London in 1692:

I was in Covent Garden when the clock struck two, when I came to Somerset-house by that it wanted a quarter of two, when I came to St. Clements it was half an hour past two, when I came to St. Dunstans it wanted a quarter of two, by Mr. Knib’s Dyal in Fleet-street it was just two, when I came to Ludgate it was half an hour past one, when I came to Bow Church, it wanted a quarter of two, by the Dyal near Stocks Market it was a quarter past two, and when I came to the Royal Exchange it wanted a quarter of two: This I aver for a Truth, and desire to know how long I was walking from Covent Garden to the Royal Exchange?

The use of "it wanted of" or "it lacked of" in reports on time was common into the 19th century, and it’s likely that the “of” construction some of us use today is just a shortening of that phrase type. But who shortened it? Which of us use it? And why?

Unfortunately, this question was not a part of the Harvard Dialect Survey, but it’s my sense from looking at online discussions of the construction and other anecdotal experience that “a quarter of” is common in the Northeast and the Midwest, and rare in the South and West. There do seem to be plenty of people all over who have never heard it (or who think of it as a “grandma” phrase).

As with most dialect features, it can be hard to pinpoint why something hangs on in one place and not others. In British English they use "half-ten' to mean 10:30. In Scotland, “back of 10” means a bit past 10—anywhere from 10:05 to 10:15. When we learn to tell time, we also have to learn the linguistic quirks of how we talk about time. The “of” construction is a quirk that I happened to pick up. What about you? Do you use “a quarter of” (or “five of” or “10 of”…) when talking about time?