Mostly works; does not account for daylight savings time, and preferences page is awkward. I'd suggest a simple list of 10-12 zones to cover the majority of users, and an advanced list (in alphabetic order) for the rest.
Edit, after a few days of use: breaks a number of sites, including flight booking at westjet.com (calendars do not work at all) and managing my Mikrotik router (all dates are incorrect by a day.)