When scheduling a dashboard you'll be asked to set a date range. The date range you choose in this window will only determine the date range for reports (within the dashboard) that are set to 'default'.
It's like changing the main date range selector (in the top right of a dashboard). If you select 'last week' in the scheduler window, all the reports (within the dashboard) will show the data for last week.
If you've overridden the main date range selector by selecting another pre-set date range or custom dates using the report level date range selector, the dates that you select in the scheduler window won't impact that report. The report level date range selector will determine the date range for a scheduled dashboard.
If you have set the date range for each report within the dashboard using the report level date range selector, it doesn't matter what date range you select. None of the reports are on default, so they won't be determined by the date range selector in the schedule dashboard window.