If you look at the sales summary for only 1 day, it shows the results in hours.
For 2 days - 1 month, it shows days or weeks.
For 2 months or more, it shows days, weeks, or months.
And it follows suit as you get a full quarter or year.
So, as I understand it, if you want to know the busiest hours for more than 1 day, you would need to input each day as opposed to a range of days.
However, to see the busiest days, all days show up in the graph no matter the range of dates. (Well, I tested up to a range of 1 1/2 years and the days still showed up on the graph; I don't know if at a certain date range the days are greyed out in the drop down menu.)