using System;

public class Program

{

    public static void Main()

    {

        string dt = "2023-08-11T00:00:00";

        string tm= "1900-01-01T11:20:00";

        var dtP = DateTime.Parse(dt);

        var tmP = DateTime.Parse(tm);

​

        var finalDt = dtP.AddHours(tmP.Hour).AddMinutes(tmP.Minute);

        Console.WriteLine(finalDt.ToString());

        //Second method

        var datePortion = dt.Split('T')[0];

        var timePortion = tm.Split('T')[1];

        var finalDt2 = DateTime.Parse(datePortion+"T"+timePortion);

        Console.WriteLine(finalDt2.ToString());

        // Third

        var finalDt3 = DateTime.ParseExact(datePortion + "T"+ timePortion, "yyyy-MM-ddTHH:mm:ss", System.Globalization.CultureInfo.InvariantCulture);

        Console.WriteLine(finalDt3.ToString());

    }

}