This is the 2727th most frequent Korean word.
약속한
"Promised."
Here, 약속한 is used as an adjective derived from the verb 약속하다 (to promise), modifying 시간을 (time).
나는 친구와 약속한 시간을 기억하고 있다.
I remember the time promised with my friend.
In this sentence, 약속한 is used as modifying 대로 (as/according to), signifying conformity to a promise.
그는 약속한 대로 행동하였다.
He acted as he had promised.
In this example, 약속한 modifies 미래 (future), indicating a future that was assured or agreed upon by mutual promise.