Código PHP:
<?
$timestamp1=mktime(17,00,00,03,18,2007); // hora, minuto, segundo, mes, dia, año
$timestamp2=mktime(22,40,00,03,19,2007);
$diferencia=$dif= $timestamp2-$timestamp1;
$diff= $diferencia / (60 * 60);
$minutos = ($diff-floor($diff))*60;
echo floor($diff).":".$minutos." horas";
?>