title: Validating DateTime format in model
body: <p>I'm adding validations to a model with a <strong>start</strong> column that is typed as <strong>datetime</strong></p>
  
  <p>Currently I only use the <strong>date_select</strong> form helper, but have left the column typed to datetime in case I decide I want to use the time value in the future.</p>
  
  <p>I am currently using:</p>
  
  <pre><code>validates :start, :presence =&gt; true
  </code></pre>
  
  <p>But I want to know if there is a :format => that will ensure I'm getting a date passed in. I know it's unlikely that someone would change the select boxes around, but I figure you can't be too careful, right?</p>