Piwik reports on days, weeks, months and years, and you can also specify a Custom Date Range in the Calendar. Click on the calendar, then on “Date Range”. Now select on the left calendar the starting date for the range, and in the right calendar the end date of the range. Piwik will process in real time the reports for the requested date range.
In the Analytics API, you can also get reports for a custom date range. For example, to request any report for the range “Feb 15th – Feb 25th”, you would use the parameters: