title: How to easily validate date select boxes in ruby on rails 3?
body: <p><code>validates_presence_of</code> doesn't work on my date field. Is there a simple way to check that user has selected a date (a birthday, for instance)?</p>
  
  <p>I'm using blank values of "Day:", "Month:", and "Year:" on my select boxes. I want to make sure that a user can't pass validation unless they select a date.</p>
  
  <pre><code> &lt;%= f.input :birthday,
               :label      =&gt; 'Birthday:',
               :as         =&gt; :date,
               :start_year =&gt; Date.current.year,
               :end_year   =&gt; Date.current.year - 106,
               :order      =&gt; [:day, :month, :year],
               :prompt     =&gt; {
                 :day   =&gt; 'Day:',
                 :month =&gt; 'Month:',
                 :year  =&gt; 'Year:'
               } %&gt;
  </code></pre>