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!!

1 Answer 1


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. 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.

  • Interesting. This isn't the answer I was expecting to see. My understanding was that に marked a specific time e.g. 来月の終わりに = 'at the end of the month', whereas で marked a period of time in which something could happen e.g. 来年で辞めるつもり = ' I plan to retire (sometime) in the next year'. I feel my interpretation is different from yours. What do you think? Commented Jul 21, 2020 at 21:28
  • I don't see much of a difference to be honest. に marks a specific time while で marks a time which includes a preceding period. However, if で can also mark a period in which something that could happen, it could be used for all future events too. But I don't think it is generally used that way. Perhaps I'm wrong about that. For example, the English sentence "I will go to Japan some day" refers to an open ended future period. Could such a sentence be marked with で in Japanese?
    – kandyman
    Commented Jul 21, 2020 at 22:16
  • Also, if で marked a period in which something might possibly happen, how might that fit with 来月で30歳になります which is a very specific time? Or am I misunderstanding your point?
    – kandyman
    Commented Jul 21, 2020 at 22:24
  • I might be misinterpreting your answer. It seems to me that you are suggesting that if I wanted to say "I will retire next Friday" I would still use で because next Friday is still after a long period of service. I may be wrong but it feels to me like I should use に in this case because I am retiring at a specific time rather than in some less well defined time that could be any point in the next year. Commented Jul 22, 2020 at 6:12
  • As for 来月で30歳になりま, I feel this works the same way. Normally one would talk about their birthday on a specific day using に, but because the speaker is being vague and just specifying the month in which the birthday happens で is more appropriate. I'm not saying I'm right about any of this. Just trying to improve my understanding. Commented Jul 22, 2020 at 6:15

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