allora proviamo a fare diversamente: non utilizziamo onBlur ma calcoliamo solo al click su un pulsante "calcola giorni" vedi se cosi ti funziona

<div class="two columns half-top">
<label for="datepicker-start">Anreise</label>
<input type="text" id="datepicker-start" name="from" class="required" />
</div>
<div class="two columns half-top">
<label for="datepicker-end">Abreise</label>
<input type="text" id="datepicker-end" name="to" class="required" /> </div>
<div class="two columns half-top">
<label for="night">Nacht</label>
<input type="text" id="night" name="night"> </div>
<input type="button" id="btncalcola" name="btncalcola" onclick="calcola()" value="calcola giorni" />


<script language="javascript">
function calcola(){
var selDate1=document.getElementById("datepicker-start").value;
var selDate2=document.getElementById("datepicker-end").value;
if (selDate1 !="" && selDate2 !=""){
var date1 =new Date(selDate1);
var date2 =new Date(selDate2);
var diff_date=date1.getTime() - date2.getTime();

diff_days = new String(diff_date/86400000);
night.value=diff_days;
}else{
night.value="";
}

}

</script>