All the requests were failing with 401 unauthorized after the timezone changes with wc1 API request.

Hi,
for the past week we able to run our code without any issues.
but i came to notice that recently the BST time zone has changed.so when i run this code inorder to retrieve the case template or even for a top level groups i was shockingly stucked by the error 401 unauthorised.
below i am proving my code foe getting the toplevel groups.can you kindly help me ASAP.since we are running in prod.
for getting toplevelgrups i have used the following

DateTime dateValue = DateTime.UtcNow; // get the datetime NOW GMT
string dataToSign = "(request-target): get " + GlobalProperties.GateWayUrl + "groups\n" + "host: " + GlobalProperties.GatewayHost + "\n" + "date: " + dateValue.ToString("R");

DateTime dateValue = DateTime.UtcNow; // get the datetime NOW GMT
DateTime localDateTime = DateTime.Now;
DateTime utcDateTime = localDateTime.ToUniversalTime();
string nzTimeZoneKey = "GMT Standard Time";
TimeZoneInfo nzTimeZone = TimeZoneInfo.FindSystemTimeZoneById(nzTimeZoneKey);
DateTime nzDateTime = TimeZoneInfo.ConvertTimeFromUtc(utcDateTime, nzTimeZone);
string authorisation = "";
string hmac = generateAuthHeader(datatoSign, GlobalProperties.APISecret);
HttpWebRequest WebReq = (HttpWebRequest)WebRequest.Create(reqEndPoint);
WebRequest.DefaultWebProxy.Credentials = CredentialCache.DefaultNetworkCredentials;
//using (WebReq as IDisposable)
//{
WebReq.Method = methodType;
WebReq.Headers.Add("Cache-Control", "no-cache");
WebReq.Date = nzDateTime;

if (methodType == "GET")

{

authorisation = "Signature keyId=\"" + ConfigurationManager.AppSettings["apikey"] + "\",algorithm=\"hmac-sha256\",headers=\"(request-target) host date\",signature=\"" + hmac + "\"";
//authorisation = "Signature keyId=\"" + ConfigurationManager.AppSettings["apikey"] + "\",algorithm=\"hmac-sha256\",headers=\"(request-target) host date\",signature=\"" + hmac + "\"";
WebReq.Headers.Add("Authorization", authorisation);
}

try
{
using (HttpWebResponse response = (HttpWebResponse)WebReq.GetResponse())
{
using (Stream answer = response.GetResponseStream())
{
StreamReader _Answer = new StreamReader(answer);
dynamic responseData = Newtonsoft.Json.JsonConvert.DeserializeObject(_Answer.ReadToEnd());
answer.Dispose(); answer.Close();
response.Dispose(); response.Close();
return responseData;
}
}
}
catch (WebException e)
{
}

@Irfan.Khan,i have also added the GMT timezone to request header but seems to b not working

Many thanks in advance.