using System;

​

public class DateTimeUtils

{

    public static DateTime RoundDown(DateTime dt, TimeSpan d)

    {

        return new DateTime((dt.Ticks / d.Ticks) * d.Ticks);

    }

}

public class Program

{

    public static void Main()

    {

        var dt1 = DateTimeUtils.RoundDown(DateTime.UtcNow, TimeSpan.FromMinutes(30));

        Console.WriteLine(dt1);

    }

}