It seems to be that if you make a report with a date field in the conditions the date changes, and the parameter check box always resets.
Generate a Report in Opportunities with a date field and a specific value for a date.
Uncheck the Parameter box.
Edit the Report
Date value = two days less than what you set
Parameter is checked again.
If you save the “wrong date” and edit again it will now be 4 days less than where you started.