Issue: Cookies getting expired before expiration time in ASP.Net

I m using below code to store cookie

if (Request.Cookies["Cookie"] == null)
                    {
                        HttpCookie CookieFavourite = new HttpCookie("Cookie1");
                        CookieFavourite.Value = id;
                        CookieFavourite.Expires.AddYears(1);
                        Response.Cookies.Add(Cookie1);
                    }
                    else 
                    {
                        Response.Cookies["CookieFavourite"].Value = Response.Cookies["CookieFavourite"].Value + "," + SomeValue;
                    }

I specity expiry date equal to 1 year. This cookie value is modified by user almost every day.

But this cookie expire after 12 hrs(approx) although i specity the expiry limit to 1 year.