Since I started learning Japanese I have always thought that when talking
about something that will occur at a certain time you either use 「に」 or
nothing. For example,

  * 来月の終わりに誕生日のパーティーをします。
  * 明日日本に行きます。

I know for some things you can either use 「に」or choose not to and other times
you can’t use 「に」 but recently I’ve noticed 「で」 popping up. For example,

  * 来月で30歳になります。
  * 来年で辞めるつもり。

Can someone explain what the rule is for using 「で」 when it comes to talking
about when something will happen!

Thank you!!
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2020-07-21T03:22:40.597
78673
2020-07-22T09:08:06.260
2020-07-21T13:32:12.100
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question
grammar meaning
で and に when you use them!
According to 'Dictionary of Basic Japanese Grammar', this usage of で
"indicates the time when something terminates or the amount of time a period
of activity has taken" (p109).

This was mentioned recently in another thread
[here](https://japanese.stackexchange.com/questions/78366/usage-
of-%E3%81%A7-and-%E3%81%BE%E3%81%A7%E3%81%AB-in-certain-verbs). The default
particle for marking a point in time is に, generally corresponding to the
English word 'at'. This indicates that the named event occurs at the named
time. It is possible to use で to mark a specific time, but doing so changes
the meaning slightly. Using で instead of に indicates that the point in time is
the end of a longer period of time which came before it. The named event is
now understood to be the culmination of a longer period which preceded it.
Your sentence 来月で30歳になります includes the understanding that it is a culmination
of a previous time period (your life up to age 29). Likewise, 来年で辞めるつもり
includes the understanding that the event (quitting the job) is the conclusion
of a longer event (working that job until the final day).

In other words, events are normally marked with に, but if you want to include
the idea of that event marking the end of a longer period of time, you can use
で.
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2020-07-21T10:38:11.057
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2020-07-22T09:08:06.260
2020-07-22T09:08:06.260
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answer