You can filter dates in two ways:
Choose one of the built-in date ranges (e.g 'Last 30 days').
The filter is always relative to today's date and adjust automatically each time the report runs.
Choose specific dates in the calendar
The filter is static and the selected dates do not change.
Set up the date filter to 'Last month' so that the report always contain the sales of the prior month. Every time the report runs, it will evaluate the filter as of the current time.
You can filter by a specific date part to include or exclude one or more values
It is sometimes useful to use the special filter value [Unknown] to include or exclude rows that don't have a value for a specific field.
Text fields can also use the special value <empty> to filter empty values.