Oh, actually we can implement via a get_timezoned_time_tempcode function.

I agree this would be useful on the forum and notifications, and perhaps the activity feed addon as well. I'm not so sure contextual dates and times would be a good idea on content though (such as publish dates for news, galleries, downloads, and so on). Since content is not a "stream", like forums, notifications, and activity feeds are... it would probably be best to leave actual dates and times for those.