expression
once upon a time; a long time ago
A literary set phrase used to begin a story, similar to 'once upon a time'. Often found in classical tales and modern narrative openings.
今は昔、竹取の翁といふ者ありけり。
Once upon a time, there was an old bamboo cutter.
今は昔、あるところに
A long time ago, in a certain place, there lived an old man and an old woman.
昔々 is the more common modern equivalent for 'once upon a time' in children's stories, while 今は昔 is more literary and classical.
A fixed phrase from classical Japanese, literally 'now is the past', used to introduce a story set in the distant past. The exact origin is uncertain, but it has been used since the Heian period.