title: Django custom field validator vs. clean
body: <p>I would like to create TodayOrLaterDateField() which would subclass DateField() field as I am using this condition in many places. The purpose of this field would be avoiding putting dates from the past.</p>
  
  <p>What is the most straightway way of doing this? I am confused with validator vs. clean method.
  I've tried with clean() but when comparing value to datetime.date.today() I am getting "compare unicode object to date" error. </p>
  
  <p>I'm using Django 1.3</p>