Good evening, Moon—If you use the present perfect, the meaning will be different. The sentence "The guests have left before five o'clock" (notice that the "o" in "o'clock" is lowercase) would only make sense in a context where the speaker is talking about a recurring event involving guests. The sentence would mean that, although the guests usually leave at five o'clock or after, they have left before five. That has happened, on at least one occasion.
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