From Longman Dictionary of Contemporary Englishonce upon a timeonce upon a timea) spokenPAST at a time in the past that you think was much better than now Once upon a time you used to be able to leave your front door unlocked. b) RFa long time ago – used at the beginning of children’s stories → once
Examples from the Corpus
once upon a time• After all, it is once upon a time.• I'd have done anything for you once upon a time.• There was, once upon a time, another book from which this kind of scientific certainty was derived.• She might never have ironed shirts, but she too had once upon a time brought Jacob little surprises, little presents.• Perhaps objects like these had been fashionable in churches once upon a time, but no longer, hence the attic.• Once upon a time children did what they were told.• One could spend a lifetime learning a small range of mountains, and once upon a time people did.• However, once upon a time the mathematician was a child too. 