Engagement call end date increased by a day.

The engagement end date has been shifted a day ahead so that when data the latest weekly data is available on Monday, rather than having to wait until the next day to view it.  AN-4150

@rocha @clintonb 
This change adjusts the date by the specified delta and returns a standard string that is used by the API.  It doesn't seem to change the purpose--getting a date centered around "now" and returning a canonical string.

Would a different method name help clarify?