2 options that I can think of... sorta.
or 3, 2 are variations of each other.
Create 2 queries, returning min and max of the dates for the complaints you're getting. Make 2 text boxes with control source = to those queries, formating the box to be MM,YYYY.
If teh query is too complicated to run in the text box, you can just make a form with 2 subforms with the queries as their record source returning the values you need. On the reports On-Load, open the form hidden, on the reports On-Close, close the form. Make 2 textboxes on the report, have them =Forms!HiddenFormName!TextBoxWithDates
These basically are the same thing.
Next option: VBA. If you can.