ーThe company's new head quarters were built in January of 2020, and their biggest rival also completed their new office building in April of the same year.
「その会社の新しい本社は2020年の1月に建てられ、彼らのライバル社も新しいオフィスを同年の４月に建てた」
It was completed in April of the same year. で「同年の4月に完成しました」と言えます。
of the same year で「同年の」
ご参考まで！
カナダ 
