Crystal Reports Passing date option parameter in command object

Message

   I am converting a Crystal 10 report to Crystal 2008. The report uses a command object. THe report has a paramter that allows the user to select for example last full month. In 2008 I am getting a error message that I have a data type mismatch between the then/else statement. Can someone help with this issue?
This is the code I am using. 

CASE
  WHEN '{?Date Range Option}'= 'Last Full Week'  
   THEN (cal.calendar_date - cal.day_of_week- 6) 
  WHEN '{?Date Range Option}'= 'Last Full Month'  
   THEN ADD_MONTHS((cal.calendar_date - cal.day_of_month)+1,-1)
  WHEN '{?Date Range Option}'= 'Custom Date Range' 
   THEN {?Start DATE}
  ELSE cal.calendar_date 
 END  AS startDate,
 CASE 
  WHEN '{?Date Range Option}'= 'Last Full Week'  
   THEN  (cal.calendar_date - cal.day_of_week) 
  WHEN '{?Date Range Option}'= 'Last Full Month'  
   THEN  (cal.calendar_date - cal.day_of_month)
  WHEN '{?Date Range Option}'= 'Custom Date Range' 
   THEN {?End DATE}
  ELSE cal.calendar_date 
 END AS endDate, 