Rails 2.1 - ActiveSupport::TimeWithZone - how to force a local time before saving ?

I am currently saving record instances starting/ending dates without any time information, (date selection from a calendar_date picker in my form)

def create @season_schedule = SeasonSchedule.new(params[:season_schedule]) .....

so I get it saved like in my DB (using config.time_zone = "Paris")

sd = ss1.starting_at_before_type_cast => "2008-10-31 23:00:00"

sd = ss1.starting_at => Sat, 01 Nov 2008 00:00:00 CET +01:00

I would like to force the starting time to be 16:00:00; how should I proceed to get it right, and changing only the time, should be in local time.... something like : @season_schedule.starting_at.set_local_time("16:00:00")

thanks for your enlightments...