In this sentence: I finished it ahead of time; it doesn't/didn't mean I had a lot of time. I think both are correct, but I hope you can correct me if I am wrong. DOESN'T means the speaker wants to say that such an event not meaning he had a lot of time is a permanent truth, so the simple present is used. DIDN'T means the speaker wants to refer to a past event and he wants to focus on the meaning of such a thing on that past occasion, so the simple past is used.
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