Hi,
I am a moderator on a Discourse forum.
It would be a great feature to see the local time of a user (at the right side of their username on each post, maybe?)
Visibility: not only for admins/moderators, but for any user.
For instance, I am located in Romania, so my timezone is gmt+2, meaning is 3 a.m. where I live.
I find it useful, when I reply to users…Other big freelancing websites, like for instance upwork, already use it.
It is not a bad idea, but IMO presence features are more useful in chat systems where you need someone to be present to have a conversation. Much of the value proposition of Discourse is that it enables working with others while being disconnected in time and space. Conversations can happen any time, not just “when someone is present”.
One interesting feature we already have is Location which is configurable by users and visible in the user card (when you click on an avatar).
I can see that in some very specific forums (for example internal company Discourse) it would be very powerful to “automatically” set location. In those case automatically making TZ visible in the card may also be interesting.
This stuff is a lot more feasible now that we have IP -> locations thanks to @nbianca’s work.
I know of a rapidly growing team with members all over the world. They use discourse extensively (both private and public instances!) and use a third party app to keep up with what time zone people are in. I think this could be useful for them.
Not really, because when anyone from that company posts, they can assume their posts will be read and acted on because they only hire awesome people who stay on top of direct mentions by their teammates. 
(Longer vacations are a far more compelling case for what you’re describing, though.)