How does time_zone_select works?

I have:     <%= form.datetime_select :datetime, :default => Time.now %>

all datetime are in UTC in the database, here is two questions: 1. how do I add the option for user to select time zone? how to user time_zone_select? 2. what is the proper way to handle that in the controller? how to add the datetime and timezone?

deeply appreciate any help.