set attribute from value of method

I am sure I am just doing this wrong, can someone point me in the right direction?

I need to set the value of effective_date to a modified format date

effective_date is an attribute of Request

class Request < ActiveRecord::Base
 
   validates_uniqueness_of :effective_date
 
   def format_date=(format_date)
     date_a = format_date.split("/")
     month, day, year = date_a
     effective_date = Date.parse("#{year}-#{month}-#{day}")
   end
 
   def format_date
     effective_date
   end
 end
 

Request.create(format_date: "04/21/2012") is not setting the value to effect_date

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