function calc_dur($data) {
	if($data['date']) return $data['duration'];
	else return $data['enddate']-$data['startdate'];
}

$table = ''
for($day=STARTTAG; $day<=$ENDTAG; $day++) { // 'k, um den richtigen Tag als Datum, timestamp, whatever zu haben muss man etwas basteln...
	$q=pg_query("SELECT * FROM activities WHERE ( startdate<=$day AND enddate>=$day+23h59m ) OR date=$day;");
	$total_dur = 0;
	while($r=pg_fetch_array($q)) {
		$total_dur += ($dur = cal_dur($r));
		$table .= "<tr><td>{$r['activity_name']}</td><td>{$r['activity_desc']}</td><td></td>$dur</tr>"
	}
	$table .= "<tr colspan=3><td>$day, $total_dur Gesamt</td></tr>"
}
echo "<table>$table</table>";