using System;

using System.Globalization;

public class Program

{

    public static void Main()

    {

        var dateStr = "12/2029";

        var date = Convert.ToDateTime($"01/{dateStr}"); //Como estava

        var date2 = DateTime.ParseExact($"{dateStr}", "MM/yyyy", CultureInfo.InvariantCulture); //Como deveria ser

        Console.WriteLine(date);

        Console.WriteLine(date.ToString("MM/yyyy"));

        Console.WriteLine(date2);

        Console.WriteLine(date2.ToString("MM/yyyy"));

    }

}