expression, adverb, noun which may take the genitive case particle 'no'
once; on one occasion; at one time
Refers to an unspecified past time or a particular occasion. Often used at the beginning of a story or anecdote.
ある時、森の中で迷子になった。
Once, I got lost in the forest.
彼はある時の出来事を思い出した。
He recalled an event from one time.
いつか means 'someday' (future) or 'one day' (past/future), while ある時 refers specifically to a past occasion.
ある日 means 'one day' and is more specific about a day, while ある時 can refer to any point in time.
Combination of the prenominal adnominal ある (a certain) and 時 (time).