How To Set  date Expiration to Today in Announcement  list 

Suppose we want to set the Expiration date to Today + 30 days after an item has been created for a list suppose Announcements list, this we can achive by using SharePoint object model.

using (SPSite site = new SPSite("http://SiteURL"))

            {

                using (SPWeb web = site.RootWeb)

                {

                    SPList list = web.Lists.TryGetList("Announcements");

                    if (list != null)

                    {

                        SPField field = list.Fields[SPBuiltInFieldId.Expires];

                        field.DefaultFormula = "=TODAY()+30";

                        field.Update();

                    }

                }

            }