using System;           

using System.Collections.Generic;

using System.Linq;  

public class Program

{

    public static void Main()

    {

        string date="2018-03-11T18:30:00+00:00";

        DateTime tempDate= Convert.ToDateTime(date);

        Console.WriteLine($"{date} = {tempDate}");

        date =tempDate.Date.AddDays(-1).ToString();

        Console.WriteLine($"{date} = {tempDate.Date.AddDays(1)}");

        DateTime tempDate1= Convert.ToDateTime(date);

        Console.WriteLine($"{date} = {Convert.ToDateTime(tempDate1)}");

    }

}