using System;

using System.Globalization;

public class Program

{

    public static void Main()

    {

        DateTimeFormatInfo fmt = new CultureInfo("en-US").DateTimeFormat;

        string dt = "2018-04-01T23:59:59.999-05:00";

        var dateTime = DateTime.Parse(dt);

        var dateTimeoffset = DateTimeOffset.Parse(dt);

        Console.WriteLine ("Date Time : " + dateTime);

        Console.WriteLine ("Date Time Offset : " + dateTimeoffset);

        Console.WriteLine ("Date Time Offset : " + DateTimeOffset.Parse(dt, fmt));

        Console.WriteLine ("Date Time Offset : " + DateTimeOffset.Parse(dt, fmt).DateTime);

    }

}