datediff question in Report Builder

I need help to to setup datediff on SQL report. Here is a logic: to get the number of days between two dates (between modify date and now). Here is my =DateDiff(day,First(Fields!modifydate.Value, "DataSet1",Today()). This one returned me an error: "The Value expression for the textrun ‘Textbox60.Paragraphs[0].TextRuns[0]’ uses an aggregate expression with the recursive/simple flag. The recursive/simple flag is not allowed for First, Last, Previous, RowNumber, RunningValue and Aggregate."

 I tried: =DateDiff(day,First(Fields!modifydate.Value, "DataSet1"),Now())  -- did not work.

Any hint?

Thank you