大学院生より標記の指摘を受けた（←ナイスな指摘をありがとう）．副詞としての「この頃」は前置詞なしの these days が普通なのに対して，「その頃，あの頃」は in those days と前置詞 in を伴う．今までまったく意識したことがなかったが，確かにと唸らされた．このチグハグは何なのだろう．
OED で these days を調べてみると，前置詞を伴わないこの形式は，かなり新しいようだ．初例が1936年となっている．その1世紀足らずの歩みを OED からの4つの例文で示すと次のようになる．
1936 R. Lehmann Weather in Streets i. v. 97 An estate like this must be a terrible problem these days.
1948 M. Dickens Joy & Josephine i. iv. 132 'Play golf?' Mr. Gray asked George, who answered: 'Not these days,' as if he ever had.
1960 S. Barstow Kind of Loving ii. iii. 181 He looks as though he's walked out of an American picture. It's all Yankeeland these days.
1981 Woman 5 Dec. 5/1 These days women are educated to expect some choice in how they spend their lives.
それより前の時代には in these days という前置詞付きの表現が普通だったようだ．後期近代英語コーパス CLMET3.0 でざっと検索してみたところ，次のような結果となった．
|Period (subcorpus size)||in these days||these days|
|1710--1780 (10,480,431 words)||10||0|
・ (from 1780--1850): Mr. Williams has been here both these days, as usual;
・ (from 1850--1920): SHAWN. Aren't we after making a good bargain, the way we're only waiting these days on Father Reilly's dispensation from the bishops, or the Court of Rome.
・ (from 1850--1920): PEGEEN. If they are itself, you've heard it these days, I'm thinking, and you walking the world telling out your story to young girls or old.
・ (from 1850--1920): "I'm as busy as Trap's wife these days;
・ (from 1850--1920): "My predecessor," said the parson, "played rather havoc with the house. The poor fellow had a dreadful struggle, I was told. You can, unfortunately, expect nothing else these days, when livings have come down so terribly in value! He was a married man--large family!"
・ (from 1850--1920): "He has no right!" Father Wolf began angrily--"By the Law of the Jungle he has no right to change his quarters without due warning. He will frighten every head of game within ten miles, and I--I have to kill for two, these days."
・ (from 1850--1920): "That is because we make you fisherman, these days. If I was you, when I come to Gloucester I would give two, three big candles for my good luck."
・ (from 1850--1920): I have literally not had time to write a line of my diary all these days.
・ (from 1850--1920): "One gets to know that birds have shadows these days. This is a bit open. Let us crawl under those bushes and talk."
・ (from 1850--1920): I do not love to think of my countrymen these days; nor to remember myself.
both や all が先行していると前置詞が不要となる等の要因はあったかもしれない．全体的には，口語的な文脈が多いようだ．in が脱落して現在の形式が現われ始めたのは19世紀後半のこととみてよさそうだ．
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