If I were to write "二〇一五年間の時に" for "during the year of 2015", would the "の時"
be redundant and/or grammatically incorrect due to the fact that the "間" in
"年間" already implies the meaning of "during this time period"?

If so, would "二〇一五年間に" or "二〇一五年の時に" perhaps be more valid alternatives?
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2016-01-14T05:10:03.783
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2021-11-23T18:06:59.950
2021-11-23T18:06:59.950
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question
grammar time
The Use of "の時” in 二〇一五年間の時に
As you said 二〇一五年間の時に is unnatural. If you say "during the year of 2015", you
say 二〇一五年に,二〇一五年の時に in Japanese but 二〇一五年間に is unnatural because 二〇一五年間 means
"for two thousand and fifteen years".
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2016-01-14T07:10:32.793
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2016-01-15T02:26:01.913
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answer
Keep in mind that the kanji 間 will drastically change the meaning.

  * 2015年に: "in (the year) 2015"
  * 2015年間に: "during the 2015 years"
  * 2015年の時に: would mean "during the time of (the year) 2015", but redundant and uncommon
  * 2015年間の時に: would mean "during the time of 2015 years", but redundant and uncommon

The latter two may appear in poems and lyrics, but in general, you should
stick to the first two expressions when you write something.
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2016-01-14T17:56:13.467
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2016-01-14T17:56:13.467
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answer