title: Validate Datetime in rails
body: <p>I am trying to validate two date fields. I want to validate that the second field is always the same date or any date after the first one.</p>
  
  <p>Example if date1 is 22/05/2011 date2 can not be 20/05/2011.</p>
  
  <p>I am using ruby on rails 3.0.7, and I am trying to do this at the model level.</p>