please advise what's the difference between 今日 and 本日 in the meaning of "today"?

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Answer

本日 is a much more formal version of 今日.

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本日　is usually found on things like menus.
for instance, 本日の焼き魚 is like "today's grilled fish"
本日 looks better to the public eye i think

Answer

As he said, "本日"" is more formal than "今日". In daily conversations, we like to use "今日" and in business scenes, we like to use "本日". Your instinct is also correct.