Addieren de référence JavaScript

strdate = "2018-03-20";
thedate = new Date(strdate);
thedate.setDate(thedate.getDate() + 1);   // 1 Tag dazu addieren
d = thedate.getDate();
m = thedate.getMonth();
y = thedate.getFullYear();
alert(d+"."+m+"."+y);
alert(thedate);
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