As it was a bit tricky to find out how this works, I want to share this script. This is how you can convert a UTC timestamp without offset and summertime into a converted timestamp using PHP. This is especially useful when extending a Calendar Plugin for example, which saves its dates in UTC-timestamps while local time is needed.

/** Calculate middle european summertime */
function utc_to_local_time($timestamp,$offset=0,$summertime=true) { 
	 $year = strftime("%Y", $timestamp); 
	$timestamp += (3600*intval($offset)); // add local offset
	
	// Calculate beginning and end of Summertime
	$initTime = mktime(2,0,0,3,31-date('w', mktime(2,0,0,3,31,$year)),$year); 
	$endTime = mktime(2,0,0,10,31-date('w', mktime(2,0,0,10,31,$year)),$year); 

    // Check if summertime, return adequatly
    if (($timestamp > $initTime && $timestamp < $endTime) && $summertime) { 
        return ($timestamp + 3600); 
    } else { 
        return ($timestamp); 
    } 
}

// example: Austria time
echo date("d.m.Y, H:i",utc_to_local_time($utc_time,1);