A custom validator for date ranges. Read the comments for directions on how to use it. Place this file in your initializers directory. I prefer this structure: config/initializers/extensions/active_model/validations/date_range.rb

date_range.rb

module ActiveModel
  module Validations
    class DateRangeValidator < ActiveModel::EachValidator
      def validate_each(record, attribute, value)
        return if value.blank?
        pass = true
 
        if comparison_date = options[:before]
          if record.send(comparison_date).present?
            pass = value < record.send(comparison_date)
            expected = 'before'
          end
        elsif comparison_date = options[:on_or_before]
          if record.send(comparison_date).present?
            pass = value <= record.send(comparison_date)
            expected = 'on or before'
          end
        elsif comparison_date = options[:after]
          if record.send(comparison_date).present?
            pass = value > record.send(comparison_date)
            expected = 'after'
          end
        elsif comparison_date = options[:on_or_after]
          if record.send(comparison_date).present?
            pass = value >= record.send(comparison_date)
            expected = 'on or after'
          end
        end
        record.errors[attribute] << (options[:message] || "must be #{expected} #{comparison_date.to_s.gsub('_', ' ')}") unless pass
      end
    end
 
    module HelperMethods
      # Validates that the specified date is valid based on the comparison date.
      #
      #   class Person < ActiveRecord::Base
      #     validates_date_range_of :start_date, before: :end_date
      #   end
      #
      # In this case, the start_date must be before the end_date.
      # If either date is not present, no error will be added.
      # Use <tt>validates_presence_of</tt> if you need to require them.
      #
      # Other options are:
      #   validates_date_range_of :start_date, on_or_before: :end_date
      #   validates_date_range_of :end_date, after: :start_date
      #   validates_date_range_of :end_date, on_or_after: :start_date
      def validates_date_range_of(*attr_names)
        validates_with DateRangeValidator, _merge_attributes(attr_names)
      end
    end
  end
end

date_range_test.rb

require File.expand_path(File.dirname(__FILE__) + "/../test_helper")
 
# we use the shoulda gem for testing.
 
class TestClass
  include ActiveModel::Validations
  include ActiveModel::Conversion
  extend ActiveModel::Naming
 
  attr_accessor :start_date, :end_date
 
  def initialize(attributes={})
    attributes && attributes.each do |name, value|
      send("#{name}=", value) if respond_to? name.to_sym
    end
  end
end
 
class DateRangeTest < ActiveSupport::TestCase
  context 'DateRange' do
    should 'not add error if main field is null' do
      TestClass.validates_date_range_of :start_date, before: :end_date
      t = TestClass.new(start_date: nil, end_date: Date.yesterday)
      t.valid?
      assert_does_not_contain t.errors.full_messages, /must be before end date/
    end
 
    context 'validates before' do
      should 'not add error if comparison field is null' do
        TestClass.validates_date_range_of :start_date, before: :end_date
        t = TestClass.new(start_date: Date.yesterday, end_date: nil)
        t.valid?
        assert_does_not_contain t.errors.full_messages, /must be before end date/
      end
 
      should 'not add error if value is before comparison field' do
        TestClass.validates_date_range_of :start_date, before: :end_date
        t = TestClass.new(start_date: Date.yesterday, end_date: Date.today)
        t.valid?
        assert_does_not_contain t.errors.full_messages, /must be before end date/
      end
 
      should 'add error if end_date is before start date' do
        TestClass.validates_date_range_of :start_date, before: :end_date
        t = TestClass.new(start_date: Date.today, end_date: Date.yesterday)
        t.valid?
        assert_contains t.errors.full_messages, /must be before end date/
      end
    end
 
    context 'validates on_or_before' do
      should 'not add error if comparison field is null' do
        TestClass.validates_date_range_of :start_date, on_or_before: :end_date
        t = TestClass.new(start_date: Date.yesterday, end_date: nil)
        t.valid?
        assert_does_not_contain t.errors.full_messages, /must be on or before end date/
      end
 
      should 'not add error if value is on same day as comparison field' do
        TestClass.validates_date_range_of :start_date, on_or_before: :end_date
        t = TestClass.new(start_date: Date.today, end_date: Date.today)
        t.valid?
        assert_does_not_contain t.errors.full_messages, /must be on or before end date/
      end
 
      should 'add error if end_date is before start date' do
        TestClass.validates_date_range_of :start_date, on_or_before: :end_date
        t = TestClass.new(start_date: Date.today, end_date: Date.yesterday)
        t.valid?
        assert_contains t.errors.full_messages, /must be on or before end date/
      end
    end
 
    context 'validates after' do
      should 'not add error if comparison field is null' do
        TestClass.validates_date_range_of :end_date, after: :start_date
        t = TestClass.new(start_date: Date.yesterday, end_date: nil)
        t.valid?
        assert_does_not_contain t.errors.full_messages, /must be after start date/
      end
 
      should 'not add error if value is after comparison field' do
        TestClass.validates_date_range_of :end_date, after: :start_date
        t = TestClass.new(start_date: Date.yesterday, end_date: Date.today)
        t.valid?
        assert_does_not_contain t.errors.full_messages, /must be after start date/
      end
 
      should 'add error if end_date is after start date' do
        TestClass.validates_date_range_of :end_date, after: :start_date
        t = TestClass.new(start_date: Date.today, end_date: Date.yesterday)
        t.valid?
        assert_contains t.errors.full_messages, /must be after start date/
      end
    end
 
    context 'validates on_or_after' do
      should 'not add error if comparison field is null' do
        TestClass.validates_date_range_of :end_date, on_or_after: :start_date
        t = TestClass.new(start_date: Date.yesterday, end_date: nil)
        t.valid?
        assert_does_not_contain t.errors.full_messages, /must be on or after start date/
      end
 
      should 'not add error if value is on same day as comparison field' do
        TestClass.validates_date_range_of :end_date, on_or_after: :start_date
        t = TestClass.new(start_date: Date.today, end_date: Date.today)
        t.valid?
        assert_does_not_contain t.errors.full_messages, /must be on or after start date/
      end
 
      should 'add error if end_date is after start date' do
        TestClass.validates_date_range_of :end_date, on_or_after: :start_date
        t = TestClass.new(start_date: Date.today, end_date: Date.yesterday)
        t.valid?
        assert_contains t.errors.full_messages, /must be on or after start date/
      end
    end
  end
end