using System;

public class Program

{

    public static void Main()

    {

        var epoch = new DateTime(2015, 1, 1, 0, 0, 0, DateTimeKind.Utc);

        var result = DateTimeOffset.UtcNow - epoch;

​

        Console.WriteLine(result);

        Console.WriteLine(result.TotalMilliseconds);

        Console.WriteLine((long)result.TotalMilliseconds);

    }

}