I'm told that to express something as "one day" in the future, it's 「有一天」. For example, 「我希望有一天能够说好华语。」"I hope that one day I can speak Chinese well."
Would you also use this to the English idiom "one day" in the past, such as "One day, the boy went for a walk"? Would that be something like 「有一天，这个男孩儿出去走了走。」?
Pardon the bad grammar.