Years

for (int year = 1900; year <= 2021; year++)
            {
                for (int mounts = 1; mounts <= 12; mounts++)
                {
                    for (int day = 1; day <= 31; day++)
                    {
                        if ((year % 4 == 0 && year % 100 != 0 || year % 400 == 0))
                        {
                            switch (mounts)
                            {
                                case 2:
                                    if (day > 29) continue;
                                    break;
                            }
                        }
                        else
                        {
                            switch (mounts)
                            {
                                case 2:
                                    if (day > 28) continue;
                                    break;
                            }
                        }
                        switch (mounts)
                        {
                            case 4:
                            case 6:
                            case 9:
                            case 11:
                                if (day > 30) continue;
                                break;
                        }
                        Console.Write($"{day:d2}.{mounts:d2}.{year}\n");

                    }
                }
            }