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I was reading a text-block in my book regarding the difference between in time and on time, and I noticed before in time, the writer (of the book) put just in 2 different examples.

Here are the sentences:

We arrived just in time to hear the Prime Minister's speech.

Our plane was on time and she was waiting for us at the airport.

We got there just in time for dinner.

I expect she will be late - the trains are never on time.

I've looked up the just in my dictionary and it collocates with in time; just in that sentence means exactly. We also know the definition of on time is punctual or neither early nor late. Is it correct to put it just before it? I wonder whether it changes the meaning of the phrase (on time).

Some people here said they are interchangeable, just in time and just on time are, if I'm not mistakenly understood.

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  • I will say "just in the nick of time" to mean that I arrive or complete something with what feels like seconds to spare. A train is either on time or it's not. "just on time" I think sounds very similar to "just in time" in fast speech and that's why some native speakers are saying it's a valid construction but I'm not sure. If a train arrives punctually then it's "on the dot” or “bang on time
    – Mari-Lou A
    Sep 30 '21 at 20:44
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"Just in time" is a very common phrase, perhaps even a set phrase. It means having achieved a deadline but with no margin to spare. Yes it means "exactly on time" but it implies a situation in which one is on the edge of failure.

In business "just in time delivery" means a model in which goods or parts are delivers the same day, perhaps even the same hour that they are needed. This will bring a failure if there is any delay in the earlier part of the logistic chain, but in many cases achieve a measure of savings.

"Just on time" is not a usual phrase. I would hesitate to say just how it changes the meaning of "on time", and I would advise avoiding it as likely to be unclear or surprising to listeners or readers.

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    just BTW, "JIT" is a common acronym for just in time, in terms of the business process as D.Siegel excellently outlines.
    – Fattie
    Sep 30 '21 at 1:25
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    Tangentially, if one's looking for a qualifier to stress the exactness aspect with "on time", then that would be "right on time". Sep 30 '21 at 9:15
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    Fwiw, 'just on time' sounds fine to me (native BrE) - e.g. the train pulls up and you look at your watch and say 'Ah, just on time!'. Just as @AngewisnolongerproudofSO says for 'right on time' - which is apparently more popular on both sides of the Atlantic, but by less margin here, and 'just' had the edge until the late 40s. (Ngram viewer)
    – OJFord
    Sep 30 '21 at 10:07
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    "Just on time" sounds like "merely on time" to me. Sep 30 '21 at 14:09
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    I could see "just on time" being used in a situation where some thing was "on time", but that this is a barely acceptable and in a place where you could replace "just" with "only". Like "We were planning to arrive early, but we were just on time.". Sep 30 '21 at 14:10
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There is a distinction between "on time" and "in time".

Something can be on time if it happens at the scheduled time. You can say "the train was on time".

However, you normally only talk about being "in time for" or "in time to" something else. For example "I was in time to catch the train". Your examples fit this pattern. However, notice that sometimes the "for" or "to" can be implicit, as in "I had to rush to the train station. Fortunately I was in time." (Here it is understood: in time to get the train.)

That means that "just" makes a lot more sense with "in time" than "on time". In contexts where you need to be in time for something, and you make it with very little time to spare, you are "just in time" for it. One doesn't often want to say something is "just on time", since there is normally no expectation that something happens before a scheduled time.

However, I suppose you might say a train was "just on time" if it would normally arrive early (and wait at the station until the scheduled time), but on this occasion it arrived at the scheduled time. Here "just in time" doesn't make so much sense - just in time for what? However, even in this case, "exactly on time" feels more natural to me.

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    That last example is an interesting one. 'just' is meaning something different here - instead of meaning 'almost not happening', it is meaning 'less than expected / unremarkable' as in: "what's that on the floor? Just an old sock". With the train example this would be: "Is the train early again? Nah, it's just on time". You can verify that it is in fact this other meaning by substituting 'just' with 'only' and seeing if it still makes sense. Both sentences do still make sense, so it must be the second meaning. Sep 30 '21 at 15:25
  • A different way to say this--"on time" compares the timing to a predefined time, whereas "in time" compares it to another event. In this sense, "just on time" doesn't make sense...the question is about the state of the subject at a particular time. "Just in time" makes more sense, because it indicates the relationship between two events. Oct 1 '21 at 14:41
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The phrase "just on time" is valid, if unusual. You could argue that "just in time" doesn't carry to "just on time", but all that is needed is the intent of 'just', meaning 'of scant significant difference'. "just on time" = "on time, but just barely"; or perhaps "on time, as expected" (eg 'just' = 'simply', 'merely', 'undeniably'. "It just is.")

I note in passing the tendency to condemn forms oneself does not use. English is an amazingly combinatorially flexible language, read "Harper's English Grammar" by John B. Opdycke, (c) 1941 as I recall, to see what I mean, in all the examples he gives.

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In American English, you might say “right on time” (with positive connotation) when someone shows up or something happens exactly on schedule, or “barely on time” (with negative connotation) where the implication is that they were nearly too late. “Just on time” does not sound right to me, but “just in time” does.

Another meaning of “just-in-time” is in computer science, where it applies to a type of interpreter, for a programing language such as Java, that translates to native machine code only as needed.

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"just in time" is a common phrase that indicates an event occurred before the necessary deadline, but with a very small margin. The phrase is referring specifically to the event that was almost missed. The "just" refers to the small margin, since "in time" simply means occurred before the deadline.

Examples:

I arrived just in time to catch the train (almost missed it!)

We managed to buy a Christmas tree in time for the holidays (with a week to spare)


"on time", however, refers less strongly to a specific deadline. It means something very similar, but you might use it more where you were just referring to an event that happened within the required period of time. It also has more of disconnected feeling and doesn't have to be specifically pointing to a deadline.

"I woke up on time"

implies perfectly "I woke up when I had planned to", whereas

Person 1: "I woke up in time ..."

feels like it needs a something else clarifying what it refers to, and would likely prompt the question

Person 2: "In time for what?"


It's much less usual to put "just" in front of "on time" to emphasise a near miss. Maybe this is because "on time" doesn't explicitly state its deadline? I'm not sure. I do know it sounds weird though. If you need to emphasise a near miss, it's safer to use "just in time" instead.

"I got to my class just on time" is much less common in English, and depending on the intonation it could sound wrong, like you had meant to say "just in time". A bit of extra emphasis on the 'just' would make it sound more natural.

There are some other more usual instances where you might hear the world "just" in front of "on time". One that jumps to mind is

I submitted my essay only just on time

Here the "only" adds that needed emphasis to the "just". You could also rearrange it to give a slightly more natural sentence structure, such as:

I only just submitted my essay on time

Honestly the differences are subtle, and a lot of the time mostly interchangeable. Like a lot of English, it's pretty arbitrary, and the only sure-fire way to get it right is spend a lot of time talking and listening with native speakers, and eventually you get an innate grasp of what sounds right and what doesn't. Here I am trying to explain it and I feel like I'm always on the verge of contradicting myself. Ha. Anyway, hope this helps.

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"just in time" is simply a common idiom and means "really really close to being in time"

"just on time" sounds very awkward and would only be used in extremely unusual cases. Do not use it, ever.

There's no mystery at all about the difference between "just in time" and "in time":

English is loaded - overloaded - with pointless filler and common idiomatic-like usages.

You can trivially type out, for English, a thousand common phrases where the meaning is all-but the same. "I'm sick of this!", "I'm sick to death of this!"

It is simply a variation.

Conceptually, the key is to grasp that English is full ("full to the gunnels" "full to overloading" "f***ing full" "soooo full" "like totally full") of slangish, idiomatic, equivalents.

Using an idiomatic variation,

    1. does not really change the sense in any meaningful way
    1. it marks the speaker as an idiomatic, native speaker

That's it. ("That's all she wrote" "We're done here" "It's a wrap" "Solid" "Word dude" "Word" "Mic-drop" etc, etc, etc.)

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    No it doesn't - it means "really really close to not being in time" (but was).
    – OJFord
    Sep 30 '21 at 10:00

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