using System;

public class Program

{

    public static void Main()

    {

        DateTime today = DateTime.Now;

        Console.WriteLine(today);

        DateTime lastDay = today.LastDayOfMonth();

        Console.WriteLine(lastDay);

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

    }

}

​

public static class DateTimeExtensions

{

    public static DateTime LastDayOfMonth(this DateTime date)

    {

        DateTime EOM = new DateTime(

            date.Year,date.Month,

            DateTime.DaysInMonth(

                date.Year,

                date.Month

            )

        );

        return EOM;

    }

}