using System;

public class Program

{

    public static int? ConvertToUnixTimestamp(DateTime? dateTime)

    {

        if (dateTime is null)

            return null;

        return (int) (dateTime - new DateTime(1970, 1, 1)).Value.TotalSeconds;

    }

    public static void Main()

    {

        var dt = DateTimeOffset.Now;

        Console.WriteLine(dt.ToUnixTimeSeconds());

        Console.WriteLine(ConvertToUnixTimestamp(DateTime.Now));

        Console.WriteLine(ConvertToUnixTimestamp(dt.DateTime));

    }

}