public class HFSendSurveyClass {
    public void SendSurvey()
    {
        
        Date hfsurvey;        
      //Get the most recently created config object.
     Config_Object__c config = [Select HFSurveyCode__c, SurveyDate__C, SOQL_Query__C from Config_Object__C
                                where Name ='HostFamily_Survey'];
       
     List<Opportunity> oppList = Database.query(config.SOQL_Query__c);        
     
     List<Account> accList = new List<Account>();             
           for(Opportunity opp:oppList)
             {
                if(opp.AccountId!= null)
                {
                  Account acc = opp.Account;
                  hfsurvey = Date.valueOf(config.SurveyDate__c);
                  if(opp.Survey_Opp_Id_Match__c== 'No')
                  {
                     string oppcode= opp.Id;
                     acc.Survey_Code__c = String.valueOf(config.HFSurveyCode__c);
                     acc.Survey_Dt__c = hfsurvey;
                     acc.Survey_Opp_Id__c = oppcode.subString(0,15);
                  }
                                     
            else  if(acc.Survey_Code__c != String.valueOf(config.HFSurveyCode__c))    
                   {
                     string oppcode= opp.Id;
                     acc.Survey_Code__c = String.valueOf(config.HFSurveyCode__c);
                     acc.Survey_Dt__c =  hfsurvey;
                     acc.Survey_Opp_Id__c = oppcode.subString(0,15);
                   }
                  
                 accList.add(acc);
                 //System.assertEquals(acc.Survey_Dt__c,Date.valueOf(config.SurveyDate__c) ) ;
                }
             }
       update accList;                            
     }    
}

The marked field is where i should be update the value frim the custome object. The surveydate in custom object is string type and the Survey_Dt__c field is a date field. and should be able to add days or change the date so i want to pass the line of code for date as a string and pick it up in my class to update it with that date.

Please suggest thanks in advance.