array(array("time"=>"12:15",
            "date"=>"2009-03-24"),
      array("time"=>"12:10",
            "date"=>"2009-03-23"),
      array("time"=>"12:00",
            "date"=>"2009-03-24"),
      array("time"=>"11:30",
            "date"=>"2009-03-24"));

array(array("time"=>"11:30",
            "date"=>"2009-03-24"),
      array("time"=>"12:10",
            "date"=>"2009-03-23"),
      array("time"=>"12:00",
            "date"=>"2009-03-24"),
      array("time"=>"12:15",
            "date"=>"2009-03-24"));

uksort($myArray, "sortThirties");

function sortThirties($a, $b)
{
    //Get the two times as timestamps relative to today
    $one = strtotime($a['time']);
    $two = strtotime($b['time']);

    //Round them down to the nearest half-hour time
    $one = $one - ($one % 1800);
    $two = $two - ($two % 1800);

    //Return the difference if times are unequal
    //If times are equal, return the difference between dates.
    return ($one == $two ? strcmp($a['date'],$b['date']) : $one - $two);
}

echo "<BR>",$a['time']," ",$b['time'];

foreach($myArray AS $a)
{
    echo "<BR>",$a['date']," ",$a['time'];
}

$to_sort = array(
                  0 => array("time"=>"12:15", "date"=>"2009-03-24"),
                  1 => array("time"=>"12:10", "date"=>"2009-03-23"),
                  2 => array("time"=>"12:00", "date"=>"2009-03-24"),
                  3 => array("time"=>"11:30", "date"=>"2009-03-24")
                );