using System;

using System.Globalization;                 

public class Program

{

    public static void Main()

    {

        string textoDoInput = "24/12/2021";

        DateTime data = DateTime.ParseExact(textoDoInput, "dd/MM/yyyy", CultureInfo.InvariantCulture);

        data = data.AddHours(DateTime.Now.Hour);

        data = data.AddMinutes(DateTime.Now.Minute);

        data = data.AddSeconds(DateTime.Now.Second);

        Console.WriteLine(data.ToString());

    }

}