Reassign Date is not a filter in dashboards. The only time reassign date shows up as an option is when the "Select Primary Record" is set to "Case" (not case with value or any others) and then the reassign date is only a selection in the columns not in filters.
The reason this is needed is to be able to exclude cases that have been reassigned to different firms. If I run a dashboard now for cases with value, I want to make sure that the reassigned cases are not included in those reports