Round DateTime to closest quarter hour
<p>How can I round a DateTime object to the closest quarter hour?</p>

<p>So: 2011-05-30 09:11:00 will be rounded to 2011-05-30 09:15:00, 2011-05-30 09:47:00 will be rounded to 2011-05-30 09:45:00 and so on.</p>
