"Date/Time validation" - class method

validates_date.rb

ActiveRecord::Base.class_eval do

  def self.validates_date(*attr_names)

    # Set the default configuration
    new_options = attr_names.extract_options!
    if new_options[:compare_time]
      configuration = { :after => Time.now.utc}
    else
      configuration = { :after => Date.today }
    end

    # Update defaults with any supplied configuration values
    configuration.update(new_options)

    # Validate each attribute, passing in the configuration
    validates_each(attr_names, configuration) do |record, attr_name, value|
      unless value.nil?
        value = new_options[:compare_time] ? value : Date.parse(value.strftime('%Y/%m/%d'))
        if configuration[:after].is_a?(Time) || configuration[:after].is_a?(Date)
          after_date = new_options[:compare_time] ? configuration[:after] : Date.parse(configuration[:after].strftime('%Y/%m/%d'))
        else
          s_date = record.send(configuration[:after])
          after_date = new_options[:compare_time] ? s_date : (s_date.present? ? Date.parse(s_date.strftime('%Y/%m/%d')) : s_date)
        end

        unless after_date.nil?
          e_msg = configuration[:after].is_a?(Time) ? configuration[:after].strftime("%B %d, %Y %H:%M%p").to_s : (configuration[:after].is_a?(Date) ? (configuration[:after]-1).strftime("%B %d, %Y").to_s : configuration[:after])
          record.errors.add(attr_name, I18n.t('must_be_after') + e_msg.to_s.humanize) if value <= after_date
        end

      end
    end
  end
end

This class method has the ability to do date and time validation checking with ActiveRecord.
The validators can be used to parse strings into Date and Time objects as well as range check.

class Project < ActiveRecord::Base
# Validates that start_date is in the future
validates_date :start_date, :o n => :create
# Validates that end_date is later than start_date
validates_date :end_date, :after => :start_date
end

class Slot < ActiveRecord::Base
# Validates that start_date is in the future
validates_date :start_date, :compare_time => true, :o n => :create
# Validates that end_date is later than start_date
validates_date :end_date, :after => :start_date, :compare_time => true
end

Example:-
p=Project.new
p.start_date = Date.today-1
p.end_date = Date.today -2
p.valid? #false
p.errors.full_messages #["Start date must be after September 04, 2010", "End date must be after Start date"]

You can customise the error messages for dates or times using I18n.