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 :

thanks for your enlightments...