MyBB Community Forums

shows all dates and times as "a month ago" "6months ago" etc.

this is better than showing dates for older posts

http://timeago.org/
https://community.mybb.com/thread-203234.html

I like exact dates and times.

It really depends on how far it goes, there's no point in transforming anything over a year.
MyBB already has relative times to some extent, but the times when you hover are too accurate, down to the minute

Do you really care which minute an event happened on when it happened five years ago?