How to change the timezone of a TimeWithZone object?

I have a model Coupon with an attribute expired_at, of class DateTime and before I save the record, I want to change the zone part of the field according to the user's choice. Say,

c = Coupon.ne开发者_C百科w
c.expired_at = DateTime.now
c.expired_at_timezone = "Arizona"
c.save!

And in coupon.rb:

class Coupon < ActiveRecord::Base
  def before_save
    # change the zone part here, leave the date and time part alone
  end
end

What I'm saying is if the admin wants the coupon expired at 2014-07-01 10:00 am, Arizona time, the expired_at stored in the DB should be something like this:

Tue, 01 Jul 2014 10:00:00 MST -07:00

Is there any way I can modify the zone part only and leave the date and time part alone?

Thanks

---

You can change the default timezone for your rails application by changing config.time_zone in your environment.rb. Usually by default it is set to UTC.

In your case each coupon has its own timezone. So you have to use a different approach. You don't have to change your save logic. You just need to change your retrieval logic. Use the in_time_zone method of Time class.

c =  Coupon.last
p c.expired_at.in_time_zone(c.expired_at_timezone)
# => Tue, 09 Mar 2010 02:06:00 MST -07:00

Otherwise you can override the expired_at method of your Coupon model.

def expired_at
  # access the current value of expired_at from attributes hash
  attributes["expired_at"].in_time_zone(self.expired_at_timezone)
end

Now you can do the following:

p Coupon.last.expired_at
# => Tue, 09 Mar 2010 02:06:00 MST -07:00

---

It's better to save all your date in UTC after, you can compare by TimeZone.