CryptoJS to C# SHA256 encryption - xamarin.forms

Question

i have been searching for a good solution where I copied code used in an API from postman and tried to use it in Xamarin forms. Problem is that there is a method in the API that generates "signatures", which they do in JS. I have tried various solutions but it does not generate the same message.
In JS =
(function () {
var timestamp = getTime();
pm.environment.set("timestamp",timestamp);
var clientId = pm.environment.get("client_id");
var secret = pm.environment.get("secret");
var sign = calcSign(clientId,secret,timestamp);
pm.environment.set('easy_sign', sign);
})();
function getTime(){
var timestamp = new Date().getTime();
return timestamp;
}
function calcSign(clientId,secret,timestamp){
var str = clientId + timestamp;
var hash = CryptoJS.HmacSHA256(str, secret);
var hashInBase64 = hash.toString();
var signUp = hashInBase64.toUpperCase();
return signUp;
}
In C# =
public void timeStamp()
{
/*
* var str = clientId + timestamp;
var hash = CryptoJS.HmacSHA256(str, secret);
var hashInBase64 = hash.toString();
var signUp = hashInBase64.toUpperCase();
return signUp;
*/
var timestamp = new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc);
var t = (DateTime.Now.ToUniversalTime() - timestamp);
time = t.ToString();
//CalcSign
var str = clientID + t;
var key = Convert.FromBase64String(secret);
//var key = Convert.FromBase64String(str);
Console.Write("key:");
prtByte(key);
var provider = new System.Security.Cryptography.HMACSHA256(key);
var hash = provider.ComputeHash(Encoding.UTF8.GetBytes(str));
Console.Write("hash:");
prtByte(hash);
var signature = Convert.ToBase64String(hash);
Console.WriteLine("signature:" + signature);
var signUp = signature.ToUpper();
sign = signUp;
}
public static void prtByte(byte[] b)
{
for (var i = 0; i < b.Length; i++)
{
Console.Write(b[i].ToString("x2"));
}
Console.WriteLine();
}
The message i get back is =
"The request time is invalid".

Answer 1

After a lot of troubleshooting and help from #Jason and #Jack Hua the problem was solved by using DateTime.UtcNow, formatting with String.Format, converting the values first to double and then to int, shown below.
var timestamp = DateTime.UtcNow - new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc);
Console.WriteLine("Timestamp = "+timestamp.ToString());
var t2 = String.Format("{0}",timestamp.TotalMilliseconds);
var t = Double.Parse(t2);
var t3 = Convert.ToInt64(t);
time =""+ t3;
Now I only need to solve my other issue "sign invalid", but that is another question. Cheers!!