validates_date :release_date, :allow_blank => true

<%= f.label :release_date, class: "control-label" %>
              <%= f.date_select :release_date, :start_year => Date.current.year, :end_year => 1970, :include_blank => true %>

<%= @miniature.release_date.strftime("%d %b %Y") unless @miniature.release_date.blank? %>

class MyController < ApplicationController
  def create
    #...
    if params[:my_model][:"release_date(2i)"] == ''
      # no month is given, insert fake month and day
      params[:my_model][:"release_date(2i)"] = '1'
      params[:my_model][:"release_date(3i)"] = '1'
      mask = 4 # 100
    elsif params[:my_model][:"release_date(3i)"] == ''
      # no day is given, insert a fake day
      params[:my_model][:"release_date(3i)"] = '1'
      mask = 6 # 110
    else
      # full-date
      mask = 7 # 111
    end
    # ...
    MyModel.create(my_model_params.merge(date_mask: mask))
    # ...
  end

  private

  def my_model_params
    params.require(:my_model).permit(:release_date, :date_mask)
  end
end

class MyModel < ActiveRecord::Base
  #...
  def release_date_display
    if self.date_mask == 4
      return self.release_date.strftime('%Y')
    elsif self.date_mask == 6
      return self.release_date.strftime('%b %Y')
    else
      return self.release_date.strftime('%F')
    end
  end
  #...
end