Can anyone give me some explanation about this sentence:

A more dramatic rise is predicted between 2030 and 2040 in Japan, by which time it is thought that the proportion of elderly people will be similar in the three countries.

What time does "by which time" refer to? 2030, 2040, or the whole period?

  • That sentence is very ambiguously worded. This is a good question, I actually think it would get constructive attention on english.stackexchange.com – RaceYouAnytime Apr 16 '17 at 14:09
  • It is just the relative word “which” that refers to “between 2030 and 2040”. – BillJ Apr 16 '17 at 14:59
  • Thanks, it might be refer to the whole period in this context. – sanba06c Apr 18 '17 at 14:27
  • This "by which time" is by the time of the predicted and more dramatic rise. – Gary Botnovcan Dec 31 '18 at 21:29

A sentence with "by which time" pretty much always is used along side a single point in time.

  • One day, by his logic, the cryptocurrency market will have its “smartphone moment”, by which time retailers might begin to find the prospect of dabbling in this space more attractive.
  • His battle with cancer first became public in November, by which time the disease had spread from his oesophagus to the main organs of kidney, lungs and liver.
  • Rahul had visited the temples only in December, by which time the manuscript of Why I Am A Hindu was with the publisher, he said.

In these examples the phrase has been used with "One day", "November", "December", indicating a single point in time. The segment with "by which time" always comes with a comma(,) before and some scenario X after the phrase.

The use of the phrase "by which time" always means that

"from now by that particular time T has been reached" the scenario X will happen/has happened(depending on the context).

My understanding is that, the example "between 2030 and 2040" also means same thing i.e.

"from now by that time 2030-2040 has been reached" the "proportion of elderly people will be similar in the three countries".

If the example was meant to indicate the period of time from 2030 to 2040, then the sentence would have been written(or the correct way to write it would be) as

"A more dramatic rise is predicted between 2030 and 2040 in Japan, during which time it is thought that the proportion of elderly people will be similar in the three countries."

Nevertheless the sentence "2030-40" could have been phrased better to avoid confusion.

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‘By which time’ refers to the whole period of 2030 to 2040. It is in the future, so we can say ‘by which time’ to refer to a time period like that, quite correctly.

If it were in the past, we would use ‘AT which time’ - so we could say for example: ‘a more dramatic rise was experienced between 1930 and 1940 at which time the elderly people were similar...’

Or we could also say ‘during which time the elderly people were ...’

But ‘at which time’ works perfectly well, when referring to a time period.


  • ‘From 5pm to 8pm I’m at my exercise class, at which time I’ll wear my hair in bunches’
  • ‘From Wednesday to Thursday I’m in the city, at which time I’ll pick up that book you wanted’
  • ‘From May to September it is warm, at which time it’s nice to go to the beach’

Note: you may substitute ‘during’ but then there’s more a sense of continuous or ongoing process:

  • ‘From September to March it’s rainy, during which time we constantly wear galoshes’.
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