DateDiff In SSRS Problem

 I am trying to do a report in SSRS and trying to calculate the number of days between the patient's re-admission and the original admission date.

Here's my formula

=FORMAT (DateDiff("DAY", Fields!ReAdmitDate.Value, Fields!AdmitDate.Value) >= 30)

tried =(DateDiff("DAY", Fields!ReAdmitDate.Value, Fields!AdmitDate.Value) >= 30)

but I am not able to resolve it, the field comes with "#Error

Data type from the DB is "DATE" , format is 06/17/2010 12:00:00 AM

Any idea what I'm missing in my code??