Excel 2003 :: Automatically Detect 30-Day Readmission?

I'm designing a workbook to track psychiatric patient data. The workbook has identical worksheets for each month of the year (Jan, Feb, Mar, etc.), plus an Admin sheet for overhead. I need to devise a formula that will automatically detect 30-day readmissions (i.e., patients being admitted to the hospital within 30 days of a prior admission) and return a "Y" for a readmit and an "N" for a discharge or first-time admission. I have tried several variations on SUMPRODUCT with multiple conditions spanning multiple sheets (using INDIRECT and a monthly worksheet index -- A1:A12 -- on the Admin worksheet), but the results always turn out...wonky. (Sometimes a correct value is returned, and sometimes not. I can't figure out what is causing this inconsistency.) The relevant ranges from the monthly worksheets are:

B3:B52 : Date of evaluation
H3:H52 : Unique patient ID number
Y3:Y52 : Patient disposition (i.e., admitted or discharged)
AC3:AC52 : Readmission status (i.e., yes or no)

The formula should only return a "Y" value if a) the patient was previously admitted within 30 days of the evaluation date, and b) the patient was also admitted on the evaluation date.

I am using Excel 2003, so SUMIFS is not available.