using static System.Console;

using System;

public class Program {

    public static void Main() => WriteLine(DateTime.Now.ToTimestamp());

}

​

public static class DateTimeExt { 

    public static readonly DateTime TimeStampStart = new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc);

    public static long ToTimestamp(this DateTime value) => (long)(value - TimeStampStart).TotalSeconds;

}