This will later be part of a longer chapter about Calendar. For now, it will be a standalone post without any introduction to Calendar — this will come later.
It is possible to select an arbitrary date range with calendar, as shown here.
Just a limitation: after more than seven days in a row, Google Calendar switches to week selection. Which means that after 7 days comes not only 14 days, but also Monday to Sunday (or Sunday to Monday, depending on your locales). And that you can’t have a ten-days calendars.