I've attached a workbook to demonstrate how this can be done using a parameter, some custom date fields, and a calculated field.
To create the custom dates, right click the date field and select "Create Custom Date...". Create a custom date for each level of detail you desire.
This is for the month level:
If you have any questions after looking at the workbook, I'd be happy to answer!
Date Level.twbx.zip 36.0 KB
Thank you so much! Worked like a charm.