"an hour early" is correct in your example sentence. an hour early = an hour earlier than the usual (normal, scheduled) time an hour late = an hour later than the usual (normal, scheduled) time If you normally arrive at the office at 7 am, you are an hour early (not 'earlier') when you arrive at 6 am and an hour late (not 'later') when you arrive at 8 am. Comparing the times without respect to your normal arrival time, you can say that if you arrived at 7 am on Tuesday and at 8 am on Wednesday, then: On Tuesday you arrived an hour earlier than on Wednesday.
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