Do you have a sample sentence? Basically it has 2 meanings: If the sentence is telling a past event, then it means *one day (uncertain) in the past*. Like "有一天, when Peter passed through the forest, he saw a strange old woman." And if the sentence is trying to predict the future, it means *one day in (the) future*. Like "Bob says, 有一天，he will be the greatest scientist over the world for sure." and "有一天，human being will evolve to another species."

means "one day"

ALL of the above is wrong! o_O haha... I think it really should be "[There] is/was(有) a(一) day(天)" example: Someday, I will meet you = There is a day [when] I will meet you. One day, a bear appeared in the forest = "There was a day [when], a bear appeared in the forest.

one day

It means one uncertain day .