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This is not a duplicate of On vs At with date and time.

Why do we say 'at' when referring to a clock-time, but 'on' when referring to a date, when they're just differently sized divisions of the larger concept of time? Why the difference?

Is it purely historical (in which case, when did this come about?), or does it key into rules of grammar?

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    Like most Whys in language, it's because we've been voting on the matter for some centuries and those are the prepositions that (for now) have won the election. – StoneyB on hiatus May 18 '16 at 15:08
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    All I have to add to @StoneyB's excellent comment is that the use of prepositions (or equivalent) is often one of the most idiosyncratic parts of a language. – Colin Fine May 18 '16 at 15:27
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    I was also curious about this, but the only answer I got was "that's the way it is". "At" suggests a little more precise location or time and "on" suggests a little larger than "at". I don't think this has to do with a grammar rule. –  May 18 '16 at 15:39
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    @StoneyB that's true, but it can be interesting to try to pick apart the reasons (as Silenus does in his excellent answer). Personally I'd much prefer an interesting question about semantics and metaphor to "Can you proof read this for me?". – Max Williams May 18 '16 at 16:25
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    @Max Williams: During the Middle English period, at and on could be used interchangeably with the hour and the calendar. – TimR May 18 '16 at 16:30
  • "On" implies a range while "at" implies a specific instant in time. – Hot Licks May 18 '16 at 16:48
  • @TimRomano - During the Middle English period there wasn't really such a thing as a specific time. – Hot Licks May 18 '16 at 16:49
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    @Hot Licks: not sure what you mean by "specific time", but the day was divided into canonical hours. – TimR May 18 '16 at 17:29
  • @TimRomano - Right, but in 1150 about the most accurate you might be is "I'll be there in the early forenoon" or some such. Saying "I'll meet you at 10:52" would be meaningless. – Hot Licks May 18 '16 at 18:08
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    @Hot Licks: Sure, but the main point still stands, that in ME we can find both "at mid-day" and "on mid-day" and other similar examples where, in modern English, we'd choose the one or the other. – TimR May 18 '16 at 18:19

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The semantics of prepositions is notoriously difficult and their uses are very idiosyncratic. Their rules of use often depend on which particular ontological domain your noun phrase is in and are heavily contingent on how we conceptualize that domain and what metaphors we employ when thinking about that domain. In this case, we're talking about temporal entities like hours, days, and months.

Here are some simple rules of thumb regarding the prepositions on and at:

  • Something about the semantics of "on" involves or suggests a two-dimensional entity (a surface).
  • Something about the semantics of "at" involves or suggests a zero-dimensional entity (a point or precise location).

See @John Lawler's answer here where he quotes Fillmore (1971):

The preposition at is said to ascribe no particular dimensionality to the referent of its associated noun; the preposition on is said to ascribe to the referent of its head noun the property of being a line or a surface; and the preposition in is said to ascribe to the referent of its head noun the notion of a bounded two-dimensional or three-dimensional space.

With this in mind, let's turn to temporal phrases like "on January 1, 2016", "in December", and "at 5:00 pm".

A larger temporal interval like a day is (for whatever mysterious psychological reasons) conceptualized as a surface, thus giving rise to the use of the preposition on.

Months, on the other hand, seem to be associated with containers (again, for mysterious psychological reasons), giving rise to the use of proposition in.

Smaller discrete temporal units like, for example, 5:00 pm, are naturally conceptualized as locations, giving rise to the use of the preposition at.

The mysterious psychological reasons that I appealed to above have to do with the way we represent time to ourselves metaphorically. Which metaphors we employ may have to do with how we've historically represented temporal entities: graphically with calendars, clocks, etc. But I know of nobody who has tried to carry out such a genealogy of temporal metaphor.

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