SharePoint使用C＃向日期字段添加天数

I am currently using an event receiver in a SharePoint site meant to onboard new employees. New user info is entered into a list called "New Hire Profile" and a second list called "Access Profile" is created based on the new hire profile list. There is a field on the employee profile called "Start Date" which is a simple SharePoint date content type. I am trying to create an calculated "Due Date" field in the access profile for tasks due on the start date. I would like to add the "StartDate" which is a date picker and "Dueindays" which is a numeric field listed on a third list "newHireTaskTemplate". The code is setup to pull the info from the fields and copy them to the new list. I am unsure how to go about adding days to the date picker date. My code currently looks like this:I am currently using an event receiver in a Sha