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one month
「一月」 has another reading with different meanings.
January
一いち月がつで五ごキロ痩せやせた。
I lost five kilos in one month.
この工事こうじはあと一いち月がつかかります。
This construction will take one more month.
一いち月がつも経たたたないうちにすっかり慣れなれました。
I got completely used to it in less than a month.