Unfortunately, Sense only allows a single billing date for the Trend summary under Bill. Under Bill, you can only view the summary from a day in one month up to the same day in the next month (i.e. June 6th to July 5th, inclusive). I have a smart meter, but even with that, the length and start days of my billing period vary by up to six days.
What I do, is I set my Sense billing date to start on the median day, the 20th. Occasionally I will readjust it when I want to compare to a specific bill, but then I change it back to the 20th.