many ways to say Today I've seen "today" translated in several ways: "今日", "今天", "本日".

Are they all official words?
What's the difference among them? Are those words more or less interchangeable in common language?
Are there any other expressions?
Are there so many ways to indicate "yesterday" and "tomorrow"?
Can you provide examples?

A similar question (and useful answers) have already been provided at &quot;Today&quot; in Spoken Singaporean Mandarin.

Thanks!
In mainland China, we say 今天 mostly. I have never heard of 本日 personally. 今日 should only be used in a formal setting like a newspaper or a TV program.