What version are you using? Also are you scripting this find or manual?
I am using version 11 of Filemaker and I manually do a find and sort.
Make sure your dates are in actual date fields--not text and then define two new summary fields. Define one to compute the Maximum of your date field and the other to Compute the Minimum.
Place these two fields at the top of your layout in a Header or similar part and you've got your date range.