How to get correct timestamp in C#

Your mistake is using new DateTime(), which returns January 1, 0001 at 00:00:00.000 instead of current date and time. The correct syntax to get current date and time is DateTime.Now, so change this:

String timeStamp = GetTimestamp(new DateTime());

to this:

String timeStamp = GetTimestamp(DateTime.Now);

var Timestamp = new DateTimeOffset(DateTime.UtcNow).ToUnixTimeSeconds();

var timestamp = DateTime.Now.ToFileTime();

//output: 132260149842749745

This is an alternative way to individuate distinct transactions. It's not unix time, but windows filetime.

From the docs:

A Windows file time is a 64-bit value that represents the number of 100- 
nanosecond intervals that have elapsed since 12:00 midnight, January 1, 1601 
A.D. (C.E.) Coordinated Universal Time (UTC).