Updating the expiry date of a cookie in ASP.NET API controller action -

struggling find answer this. set cookie allows me track visitors shopping cart. setting cookie fine can't find way of updating expiry date, each time user cart related updates 90 days in future. code below. addtocart api method in question.

[actionname("addtocart")] [httpget] public httpresponsemessage addtocart(int id) {     httpresponsemessage msg = new httpresponsemessage();     cookieheadervalue header = request.headers.getcookies("visitor").firstordefault();     visitorviewmodel visitor;      if (header == null)     {         visitor = _visitormanager.create().toviewmodel();         header = new cookieheadervalue("visitor", visitor.cookie);         header.expires = datetime.now.adddays(90);         msg.headers.addcookies(new cookieheadervalue[] { header });     }     else     {         visitor = _visitormanager.get(header["visitor"].value).toviewmodel();         header.expires = datetime.now.adddays(90);         msg.headers.addcookies(new cookieheadervalue[] { header });     }      _cartmanager.addtocart(id, visitor.id);      return msg;  } 

if user has not got cookie yet adds cookie correctly, if cookie exists end second cookie kind of i'd expect based on code. can't find way update existing client cookie.

in end path of cookie. ensuring set path "/" in setting , retrieving of cookie cookie updated correctly.