I wonder if a possible solution could be to implement the same scheme used at gmail, ohloh and other sites, whose show the elapsed time since the given event (1 hour ago, 1 day 15 hours ago, etc.)

That way, you are timezone independent

Is it possible to add this feature in the roadmap?

another vote for this feature - we use one mantis install across users in 3 countries. I used putenv("TZ=xxx"); in the config as suggested elsewhere on mantisbt.org, but allowing each user to set their own client timezone would be fantastic.

There is a time zone setting in "My Account" -> "Preferences" since version 1.2.x