As a teacher, my obsession is Point of View and Time.
I cover them as inseparable — until my students are on the point of thinking, Not again.
My belief is that unless you are absolutely secure in knowing when your narrator is speaking, in relation to the events they are telling, then your tone is likely to wobble.
Third person omniscient narrators are also, implicitly, situated in time. (Usually.)
The easiest solution to securing this is to say to yourself something like this, My third person omniscient narrator is looking back soon after the final event in the timeline, and they are telling the story not in chronological order but in the most satisfying order for the reader.
Fine. That’ll do.
Or to say to yourself, My first person narrator is at this midpoint in their timeline, and everything they say is appropriate to their attitude at that precise moment. Later on, I will jump forwards in time, and their attitude will have changed.
If this means they are always narrating in the breathless first person singular present tense, then they won’t have much time for retrospect or judgement. They won’t have lyrical distance, except during interludes in the action when they’re looking back on earlier moments.
However, because of this narrator’s efficient selection of significant scenes and details, and their occasional use of dramatic irony, it may be that — at base — they are secretly narrating from a point after all the action is concluded, but are mimicking themselves giving a moment-by-moment account.
You couldn’t construct an ordered account of climbing a mountain, with appropriate flashbacks, whilst climbing it.
If all of this sounds like overthinking, that’s fine. Take it from me, you need to be able to place your narrator precisely in time.
That way, the reader will know where they are in time, and what the story is meant to mean — is it a conclusive statement, or is it questionable, because still progressing?
What follows is an exercise I do early on in the term, with the whole group, but it’s also one you can do by yourself.
I introduce it by saying something like —
It is important that the reader understands where they are in time, in relation to what has gone before.
And adding —
The sooner they learn this, within a sentence, the better.
You can give the students or yourself some ingredients for the story — some character names, some places, some objects, some mood words — but the exercise itself should help you shape a narrative.
When everyone is settled, and has pen and paper or laptop, I read out the first sentence beginning. This is what they have to start with. These words must go on the page, and then they continue from there —
1. Late that long August…
I let the students write for three minutes between each of the sentence beginnings. If someone hasn’t started until two minutes in, I’ll wait a little longer. But from then on, I’ll give them a new prompt — which must be included — every three minutes.
These are the prompts.
2. It was on a hot Tuesday evening…
3. By the end of the night…
4. The following morning, everything was different…
5. Six years before this summer…
6. But that seemed a long time ago, and since then…
7. September finally came, and…
8. It rained that Sunday…
9. At exactly seven minutes past two in the afternoon…
10. Ten seconds later…
11. S/He or I or We had to make an instant decision…
12. The period that followed was difficult for everybody…
You can add to this, or take away.
What I’m doing here is guiding the writers toward dilating and then concentrating the time of their story — to going from a lyrical account of a remembered period to a race against the clock.
It may be that some of them have not often used, or are not comfortable with, one or other end of this scale (months and years or moments).
After everyone had finished writing, I congratulate everyone on having produced so much. And say I hope they’ve learned something from it.
My assertion is that readers hardly even perceive these clauses, these placements. They just clock them as time-markers and immediately focus on the content of the next words. On the story time, in which they are in a certain place.
But if these — or these kinds — of time-makers are missing, then the reader will start to waste their own time in working out what is going on, and how it relates to what has happened before.
They become confused, and lose trust in the writer.
Only Marcel Proust should have to go in search of lost time.

What's your opinion on using a second character's point of view (using some separate interleaved chapters) to provide the effect of "secretly narrating from a point after all the action is concluded"?
Toby’s novel _Patience_ is a lovely example of how this works in practice when done well. The narrator (first person, past tense, a young man trapped in his body by chronic illness) writes with enough time after the novel’s main events to have experienced critical distance, and so the reader is given passages that mix timescales in a pattern that one learns early on and can follow the rest of the book. It goes something like this:
“L happened, thus M, and causing N, which was just like when A to B to C, and although back then I thought D, instead today I decided O and still carry O with me even through X and Y. I was thus able to survive the inevitable P, causing Q, which happened the rest of the next day, until at last R came around, bringing S.”
It swings around and around but the dominant narrative momentum of the LMNOPQR middle sequence of events is the narrative focus, and we have enough glimpse of ABCDE… and …WXYZ that we experience the narrator as a fuller, wiser character.