This is a bug https://github.com/NodeBB/NodeBB/issues/7814
How psychobunny visits the site or 'about a month ago'
It's because I "appear offline" as my user setting.
If you think about it, it makes sense, because if I am appearing offline you can check to see if I'm actually online by looking at the "last online" (which would say "1 minute ago")
That said, I think that "last online" should be updated when the user does something public such as reply to a post