$A.get("e.force:navigateToURL").setParams(
    {"url": "/apex/Accountsummary"}).fire();

returns an error that the page does not exist

from: https://trailhead.salesforce.com/modules/visualforce_fundamentals/units/visualforce_standard_controllers

We would like to set a default date for a Due date field as "Today plus 3 business days".
Is this possible? If so, what would the formula be?