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In this
[article](https://www3.nhk.or.jp/news/easy/k10012858851000/k10012858851000.html),
first sentence in the second paragraph:

> この事故{じこ}から、[１０日]{とおか}で２０年{ねん}になりました。

My interpretation is that

> On the 10th, it will have been 20 years since the incident.

To specify the exact date, they use で. But why で instead of に? I've heard で to
be understood as "by means of" for some action (say as in this [で v に
answer](https://japanese.stackexchange.com/questions/60/particles-%E3%81%AB-
vs-%E3%81%A7)) and it doesn't seem to be too fitting here. I expected に to be
used just like the second part, where it's used as a target of なる, for some
like

> [１０日]{とおか}に２０年{ねん}になりました
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meaning particle-に particle-で
Using で instead of に when specifying a day
So the questions and answers provided by Eddie Kal in the comments pretty much
answer your question, but I can see how it might not be immediately obviously
how they apply in this case.

First of all, just to be clear, `で` can be used for expressing time/timing in
certain cases. You will see this listed as a possible definition in pretty
much any dictionary, including [Jisho](https://jisho.org/search/%E3%81%A7). In
particular, as mentioned by [the first
answer](https://japanese.stackexchange.com/a/62192/7705) and alluded to [by
the second answer](https://japanese.stackexchange.com/questions/83629/the-
difference-between-%E3%81%AB-and-%E3%81%A7-when-indicating-the-time-when-
something-terminate) provided by Eddie Kal, `で` carries a fairly strong
connotation of something coming to an end or resolving at a specific time.

In regard to how that applies to this specific case, you said in the comments:

> Those are regarding verbs that terminate, almost like an "until", while in
> this case there's no termination or deadlines, and only the day itself is
> significant

Arguably the process of becoming exactly 20 years old is arguably something
that requires exactly 20 years and then terminates. Looking at it that way,
this seems like an appropriate usage of `で`. If that doesn't seem intuitive to
you, you can also just think of `で` as emphasizing the significance of the
specific time.

> この事故から、１０日で２０年になりました。
>
> On the 10th, it will have been 20 years since the accident to the day.

`で` here is perhaps not quite as strong as English's `to the day`, but it's
the closest thing I could come up with.
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