Datetime format

You can let DateTime structure parse the date (and cope with it in the best-try manner):

try
{

    string raw = "05/Jul/2011";
    DateTime dt = DateTime.Parse(raw);
    string formatted = dt.ToString("dd/MM/yyyy");

    Console.WriteLine(formatted);

}
catch (System.Exception ex)
{
    Console.WriteLine(ex);
}

Make sure that all exceptions are handled because user is free to enter garbage in the text box. You can enforce input string to be in the format stated in your question by matching it against a regular expression:

try
{

    string raw = "05/Jul/2011";

    Regex reg = new Regex(@"\d[\d]/(Jan|Feb|Mar|Apr|May|Jun|Jul|Aug|Sep|Oct|Nov|Dec)/\d\d[\d\d]");

    if (reg.IsMatch(raw))
    {
        DateTime dt = DateTime.Parse(raw);
        string formatted = dt.ToString("dd/MM/yyyy");

        Console.WriteLine(formatted);
    }
    else
    {
        Console.WriteLine("Incorrect input string.");
    }

}
catch (System.Exception ex)
{
    Console.WriteLine(ex);
}

In this code, if you change string raw = "05/07/2011", that would print out "Incorrect input string." without exception.

Zoran