About at about
But Monkey, I hear you cry, are not all rules of grammar somewhat arbitrary. Well yes, possibly -- Chomskian minimal project aside. They are at least conventional. But of course they do generally obey the internal logic of the language.
So a rule we often see is that one may not write "at about 8pm" in sentences like "The car crashed into the wall at about 8pm". One must write "The car crashed into the wall about 8pm". This sentence reads awkwardly. I'm not going to go into a huge digression on why it does, but certainly English prefers adverbial phrases of time to be fronted, so "About 8pm, the car crashed into the wall" reads much more comfortably.
So the reason given for the rule is that "at" is specific and "about" is vague. When we say "at an hour" we mean "precisely at that hour". The first is simply incorrect. "At" is a preposition that indicates position exclusive of other positions. It doesn't have to be specific or accurate. Nothing about it suggests that. It's in the nature of most phrases it is used in that it seems binary: "at home" means at home and not "not at home". However, "at home" actually means at home and nowhere else; "at work" means at work and nowhere else. "At work" in particular is quite a vague sort of concept. We do not mean it to say "at my workplace". We mean it to say "at the doing of my work and not doing anything else".
So "at about 7pm" means "at about 7pm and at no other time". "At" is not specific; it is exclusive. Compare "you or some other person". This is perfectly acceptable in English.
You're not convinced, I know. So let's look at a perfectly acceptable English sentence: "I will come at sevenish." No one will argue this is ungrammatical, because it plainly is. Yet the time specified is not pinpoint, is it?
Let's try another: "he killed her at some time after 9pm". Again, the time is not pinpoint. Indeed, it's a much broader sweep of time than we generally mean by "about". It's "the entire time after 9pm until now". But we can still use "at" (although it's not obligatory, of course).
Here's the killer though. You're going to like this.
Let's say you bring a lamb to the Ekka and it's weighed. The weigh-in dude says to you "that lamb is roughly 9kg". All good, right? We use "roughly" in this instance as precisely a synonym for "about".
"That lamb weighed in at roughly 9kg." is perfectly grammatical. Wait, hell no! That's "at" with a nonpinpoint measure. That surely cannot be.
But it can. And it is. And yes, I did just begin three sentences with conjunctions. Dealwithit.jpg.
Indeed, you cannot say "*That lamb weighed in roughly 9kg".
Okay, it's at about this time that the most diehard pedant will be gnashing their (or his or her, whatever you prefer) teeth because you can't really use "roughly" in the sentence I began with, so "about" and "roughly" are not perfect substitutes. But really, they don't have to be. The pedant claims that "at" cannot be used with imprecise terms. I just showed it can. So now the pedant is left with "at" cannot be used with imprecise terms to do with time, except, erm, "sevenish" and the like. Or that whole "after 9pm" thing. And I just say bullshit, take my bow and adieu, dear friends.