at an early date = early in the lifetime of.... It could be the lifetime of anything. In the example above it refers to the "lifetime" of what we know about honey. Since our knowledge of honey began in the past, so all early dates will be in the past. If something begins now, then an early date would be the near future.

Yes, you are right, most of "at an early date" is used in the Past Tense. For the case of the Past Tense you must write the translation like that "в непродолжительном времени". If the "at an early date" is used in the Present or Future Tense, that the translation should be "в ближайшем будущем". Английский язык обладает одной интересной особенностью, вы должны постоянно анализировать контекст текста. Например, если я хочу выразить то же самое слово "в ближайшем будущем" по-английски, то почему-то получается так:"I am looking forward to hearing from you very soon (I expect your early reply) (I hope to hear from you very soon)" или в такой форме "I would appreciate a reply at an early date". Словари тоже могут ошибаться! В данном случае "Even at a very early date it was recognized that honey tends to prevent putrefaction" здесь фраза "at a very early date" очень напоминает другую фразу "at an earlier date" (в сжатые сроки, за короткий период). И здесь ничего похожее про что-либо будущее или о намерении в будущем.

hmm... I'm just curious about your perception of the usage of "at an early date" that led me to looking for its meaning in the web. here's what Thesaurus says: at an early date: soon; some day soon. The note said, "Please call me at an early date." "You are expected to return the form to the office at an early date." I'm just thinking that "even at a very early date" could imply both past or future action. :)