there have been calendar MODs and extesions over the years but apparently they are not popular enough to keep updating etc by their authors.
the main reason a calendar is not part of the phpbb code is that the original and still valid concept of phpbb is to make a basic bulletin board system. that requires forums and posts and not much else.
anything else is an addon. that includes a calendar.
I would assume the reason why you can't find too much about a current calendar extension is because there is apparently not that much interest in one.
I'm baaaaaccckkkk. still doing work on donation basis. PM your needs.
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