Question

How can I round a DateTime object to the closest quarter hour?

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.

Answer 1

You could extract the minutes divide them by 15, round them and multiply them by 15 again.
Something like

// round the "minutes"
$quarter = round(date('i', $yourdate) / 15) * 15;
// get the new timestamp
$roundeddate = mktime(date("H",$yourdate), $quarter, date("s",$yourdate), date("n",$yourdate), date("j",$yourdate), date("Y",$yourdate));