using System;

using System.Globalization;

public class Program

{

    public static void Main()

    {

        System.Globalization.CultureInfo provider = System.Globalization.CultureInfo.InvariantCulture;

​

        string dvString  = "1Янв";

        //string dvString  = "10Янв";

        string format = "dMMM";  

        /* ошибка:

        string dvString  = "1Янв";

        string format = "ddMMM";  

        */

      DateTime result;

      bool check;

      check = DateTime.TryParseExact(dvString, format, CultureInfo.GetCultureInfo("ru-RU"), DateTimeStyles.None, out result);

        if (!check)

                            {

                                throw new Exception("Ошибка при определении отчетных суток. dvString{dvString}");

                            }

        Console.WriteLine (result);

    }

​

}