Here's a sentence written in my textbook.
The first time I ever went to a club must have been 5 years ago.
I'd like to make another sentence using this like following.
It must have been by the time I had been working just for a few weeks that a memorable incident happened.
I really do know I have been getting a huge help here. Thank you all seriously.
"It must have been WHEN I had been working FOR JUST a few weeks..."
"by the time" is awkward in your example; it adds nothing to comprehension (it sounds as if you had been anticipating that event for three weeks.)
You need simply "when" (or "after", if that is what you meant).
And "just" modifies "a few weeks", it does not modify "for a few weeks". This glitch would not be noticed in speech, but it matters in written English. Compare:
Just I had been working for a few weeks (nobody else was working—this construction is rare; you'd more likely say "I was working alone...")
I had just been working for a few weeks (you had recently finished three weeks of work)
I had been just working for three weeks (you did nothing but work for those three weeks)
I had been working just for three weeks (you expected to not be working longer than three weeks)
I had been working for just three weeks (merely three weeks, only three weeks; a relatively short time compared to how long you expected to work there)
The last one expresses best what you seem to mean.
An idiomatic version of:
It must have been by the time I had been working just for a few weeks that a memorable incident happened.
might be:
I must have been working (there) for just a few weeks when the incident happened.
This form of "must have been" implies that your memory is not absolutely perfect, and you're orienting yourself temporally (doing the date-math in your head, so to speak) even as you speak. You're figuring out the chronology while you're talking.
I do understand what you're trying to say in the sentence, but it isn't quite right.
The first thing I notice is a bit of a contradiction. Note the bolded parts.
It must have been by the time I had been working just for a few weeks that a memorable incident happened.
Used here, "must have been," particularly but not exclusively with "by the time," makes it sound like it was a long time, and "just a few" makes it sound like it was very little time. The former doesn't necessarily mean that that, but it tends to have a fairly sentimental feeling to it, and that tends to highlight duration.
On top of that, using "that" here seems a little awkward. I would probably prefer "when," because we're talking about time.
I think a better way to phrase that sentence might be,
I had been working for just a few weeks when a memorable incident happened.
Alternatively,
It must have been five years since I first went to a club.
As far as an "it...that..." construction is concerned, I can't think of many times when that really sounds natural. A few that do occur to me off-hand are,
- It has been so long that I've forgotten.
- It is interesting that you say such a thing.
Although each of these are cases where the sentence could be very (perhaps more) comfortably formed without the word "that" in them, so I'm not sure how much they even count.
