Need To Blank Calculated Date Field If Another Date Field Has No Date Entered

OK, I see now that you did do it, but incorrectly. Use this:

var s = this.getField("DatePerformed").valueAsString;
if (s=="") event.value = "";
else {
	var d = util.scand("mm/dd/yyyy", s);
	d.setMonth(d.getMonth() + 3);
	event.value = util.printd("mm/dd/yyyy", d);
}