expression
it's already too late
A set phrase indicating that the opportunity has passed and nothing can be done now. Often used with a tone of regret or resignation.
気づいたときには時すでに遅しだった。
By the time I realized, it was already too late.
時すでに遅し、彼はもう去った後だった。
It was already too late; he had already left.
Both express that something is too late, but 後の祭り is more colloquial and often used for personal regrets, while 時すでに遅し is a more literary, dramatic phrase.
A classical Japanese expression combining 時 (time), すでに (already), and the classical adjective 遅し (late). The phrase has been used since at least the Edo period in literature and drama.