Display a "Best sellers of the month"

Hi,

There is no automatic system for that.
You can have best sellers without a time period.
But there is no easy way to know the best sellers for the last 30 days. That would require to dynamically compute the number for all the products every day based on the orders of the last 30 days.
With the best sellers without a time period like there is already, it's much easier as we can just increment the counter when a sale is made.

What people often do is to actually assign the products they want to show in a specific unpublished category and configure their "best seller" module to just display the products of that category. That way, you can update the products displayed by the module when you want.