function jeff_event_pre_submission_filter($post_data, $form, $entry ) {

    if( $form["id"] != 9 )
       return $post_data;

    $time_start = strtotime( $entry[2] . ' ' . $entry[3] );

    if( $entry[9] == 'Yes') {
        #is multi day
        $time_end = strtotime( $entry[10] . ' ' . $entry[4] );
    } else {
        #one day event
        $time_end = strtotime( $entry[2] . ' ' . $entry[4] );
    }

    #_DURATION
    $_POST['input_43'] = duration( $time_start, $time_end );

    return $post_data;

}

function duration( $start, $end ) {

    $seconds = $end - $start;

    $days = floor( $seconds / 60 / 60 / 24 );
    $hours = $seconds / 60 / 60 % 24;
    $mins = $seconds / 60 % 60;
    $secs = $seconds % 60;

    $duration = array();

    if( $days > 0 )
        $duration['days'] = $days;

    if( $hours > 0 )
        $duration['hours'] = $hours;

    if( $mins > 0 )
        $duration['minutes'] = $mins;

    if( count($duration) == 0 )
        $duration['minutes'] = 0;

    return $duration;
}