title: Range of DateTime with different resolution than one Day
body: <p>how can i get a list of DateTime objects which match the following criteria:</p>
  
  <ul>
  <li>they are between two instances of DateTime</li>
  <li>they fall on a fraction of an hour/min.  eg. they are a full quarter of an hour</li>
  </ul>
  
  <p>with active support a possible solution is:</p>
  
  <pre><code>(my_datetime_ob_a.to_i .. my_datetime_ob_b.to_i).each { |timestamp|
    puts timestamp if (timestamp % (3600/4) == 0)
  }
  </code></pre>
  
  <p>this is not nice: too many iterations and too much conversion (you would need to reconvert timestamp to a DateTime object.</p>