Calculating date using DateAdd control

I have a challenge with DetailsView Server Control - ASP.NET.
I have a database table in an SQL Server Express Edition. Below is the table structure:
Column Name                 | Data Type
ClientName                     | varchar(50)
MembershipNo                | varchar(10)
RegistrationDate             | date
YearsRegisteredFor         | int
ExpiryDate                       | date
A_WeekB4Expiry              | date< ...