Crystal Reports Help with a Code

Message

   I work in a hospital and I am trying to write a report that shows readmissions.  the formula that I am using right now looks like this:

{SSVIEW_PATIENT_DATA.ADMIT_PROGRAM_VALUE} <> "PREADMIT" and
{SSVIEW_PATIENT_DATA.DATE_OF_DISCHARGE} in {?Start Date} to {?End Date}

I have it designed to look 365 days in the past.  What I really need to look at is the previous discharge date and have tried this:

{SSVIEW_PATIENT_DATA.ADMIT_PROGRAM_VALUE} <> "PREADMIT" and
If previous({SSVIEW_PATIENT_DATA.DATE_OF_DISCHARGE}) in {?Start Date} to {?End Date} then true else false

it tells me that this function can not be used because it must be evaluated later.  I need to be able to pull the date range from previous discharge.  Please Help!!!