at that time; in those days; referring to the time when an event occurred.
当時はまだスマホが普及していなかった。
At that time, smartphones had not yet become widespread.
彼は当時、大学生だった。
He was a university student at that time.
当時の新聞記事を図書館で調べた。
I checked newspaper articles from that time at the library.