It seems like an adjective in those phrases. Are you sure that 'early' is an adverb? Early是副词，a few hours early不构成词组，是中国式的英语。 请看下面两个例句： i came three hours earlier than the others.
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我比其他人早来三小时。 the ship arrived 30 minutes early.
会议通知中的early bird deadline是什么意思在国际会议的注册通知中，经常会提到一个术语“early bird deadline”，它指的是提前注册的截止日期。设立这个日期的主要目的.
