Q: I use expressions like “ten of five” to mean 4:50 and “a quarter to ten” to mean 9:45. I’m from Connecticut, but people from other parts of the country have no idea what I mean. Where did this method of telling time come from?
A: Your using “of” to tell time is perfectly legitimate. And it’s by no means confined to Connecticut, either. I can tell you that it’s also common in the Midwest, where I come from.
The earliest sense of the preposition “of” was “away” or “away from,” according to the Oxford English Dictionary. (This sense is retained in the spelling “off.”) When we say it’s “ten of five,” we mean that five o’clock is ten minutes away.
Your method of telling time has been common since the early 19th century, though the preposition “to” has been used in the same way since roughly the year 1000.
In North American, Scottish, and Irish English, “of” is commonly used “in expressing the time: from or before (a specified hour),” the OED says. Here are the dates and partial citations given.
1817: “At 15 minutes of 10 a.m.”
1857: “Five minutes of nine.”
1912: “It is a quarter of twelve.”
1956 (from Patricia Highsmith’s novel The Blunderer): “His watch said twenty of six.”
1995 (from a story by T. C. Boyle): “It was a quarter of one.”
The usage is also seen with the hour unstated but understood, as in Ira Levin’s novel The Boys from Brazil (1976): “I leave at five of.”
So don’t be apologetic. And now that it’s a quarter of one, I’m quitting for lunch.
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