PDF Form: Date Validation script

OK, then you can use this code as the custom Validation script of the Delivery Date field:

var orderDateString = this.getField("Order Date").valueAsString;
if (event.value && orderDateString!="") {
	var orderDate = util.scand("dd/mm/yyyy", orderDateString);
	var deliveryDate = util.scand("dd/mm/yyyy", event.value);
	
	if (deliveryDate.getTime()<=orderDate.getTime()) {
		app.alert("Error! The Delivery Date is less than or equal to the Order Date.");
		event.rc = false;
	}	
}

Note that you'll also need to apply a similar script to the Order Date field, to validate its value against Delivery Date, and the same with Delivery Date-Operational Date.