DateTime from string using string format

[finesoul]

    private DateTime GetPublicationDate()
    {
        DateTime publDate;
        string publDateStr = Request.Params["PublicationDate"];
        if (string.IsNullOrEmpty(publDateStr))
            throw new ApplicationException("You must send query string parameter 'PublicationDate'");

        if (!DateTime.TryParseExact(publDateStr, "yyyyMMdd", null, DateTimeStyles.None, out publDate))
            throw new ApplicationException("Invalid 'PublicationDate' format");
        return publDate;
    }