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In the following sentence: "It came into existence on or by 30 September." What does "by 30 September" mean?
It means "exactly on September 30, or on September 30, it was in existence." The second half of this means that it could have come into existence prior to Sept 30, but the speaker doesn't know exactly when. If the question is more about what hour it came into existence, that is purposefully left vague by this construction.