Simple date validator for Rails 3.

date_validator.rb

# Validator:
# /app/validators/date_validator.rb
class DateValidator < ActiveModel::EachValidator
  
  def validate_each(record, attribute, value)
    unless value.to_s =~ /^\d{4}-\d{2}-\d{2}$/i
      record.errors[attribute] << (options[:message] || "is an invalid date") 
    end
  end
  
end

# Use:
# /app/models/post.rb

class Post < ActiveRecord::Base
  validates :published_on, :date => true
end