Cookie Persistence : cannot set expiry date - asp.net

Question

I am attempting to change an MVC web site that was storing some info in Session to store it in a cookie but am having some issues with the expiry of the cookie.
Scenario
While logging in to the site a cookie is created and added to the current HttpContext Response cookies collection. The code piece used to write the cookie is as follows :
HttpCookie userCredsCookie = _context.Response.Cookies.Get(Constants.Web.UserCredentialsCookieName) ?? new HttpCookie(Constants.Web.UserCredentialsCookieName);
userCredsCookie.HttpOnly = true;
userCredsCookie.Secure = true;
userCredsCookie.Value = user.ID.ToString();
userCredsCookie.Expires = DateTime.Now.AddMinutes(user.CompanySessionTimeout + user.TimezoneOffset);
_context.Response.Cookies.Add(userCredsCookie);
I have experimented with removal of the line that sets the expiry of the cookie and have found that the cookie is generated and stored at the client as expected.
When i inspect the response from the server using Fiddler i can see that the userCreds cookie is being returned :
Response sent 111 bytes of Cookie data:
Set-Cookie: UserCreds=e2ce8200-fb38-45b9-8aec-4d93e6640a84; expires=Mon, 09-Oct-2017 16:20:43 GMT; path=/; secure; HttpOnly
when the expiry is not set the response has :
Response sent 72 bytes of Cookie data:
Set-Cookie: UserCreds=e2ce8200-fb38-45b9-8aec-4d93e6640a84; path=/; secure; HttpOnly
As long as the expiry is not set, the cookie is accepted by the browser and sent on the subsequent requests made from the browser.
It feels like the problem is due to the format of the date in the response but i cannot find anything to allow me to change that format.
Any help would be greatly appreciated.
Regards
Monty
Update - Possible cause located
I have found that the problem i am having is to do with timing. It seems that my dev pc may be processing the responses from the ajax call that is setting the cookie too fast (??? is that even possible ???).
While i was debugging the front end code to ensure things look fine from that perspective i found that by placing a break point just before the line which was setting the window.location to a url that is in the result of the ajax call, the subsequent call was made with the cookie being on the request.
Perhaps, by causing the processing of the callback to pause, gave the system time to persist the cookie. I assume the process of persisting a cookie is to do a validation check then store the data. That would mean that it takes a little longer to validate the cookie when their is an expiry date set. Perhaps the reason why a cookie with no expiry set was persisting but ones with an expiry were not was cause the load of a new url interrupted the process, thus causing the cookie to not get stored.
By adding a 1 millisecond delay (yep, just 1 millisecond) in the success handler of the ajax call i was able to successfully have the cookie persisted to the browser and included in the call made to load a return url that is in the ajax response.
Does this sound even possible?

Answer 1

try with this code
int result = Convert.ToInt32(user.CompanySessionTimeout.ToString()) + Convert.ToInt32(user.TimezoneOffset)
and then pass
userCredsCookie.Expires = DateTime.Now.AddMinutes(result);