[翻译]C#.我该如何分析和转换日期时间的到RFC 822的日期，时间格式(How do I parse and convert DateTime’s to the RFC 822 date-time format?)

转载自：https://stackoverflow.com/questions/284775/how-do-i-parse-and-convert-datetime-s-to-the-rfc-822-date-time-format

问：RFC822 日期-时间被很多应用到很多规范中，比如 RSS格式 ；在.NET中，我怎样将DateTime结构转换成等效RFC822日期时间格式

字符串展现，并且又能解析此字符串(string),重新用DateTime结构的呈现呢？

 

解决方法1：

DateTime today = DateTime.Now;
String rfc822 = today.ToString("r");
Console.WriteLine("RFC-822 date: {0}", rfc822);

DateTime parsedRFC822 = DateTime.Parse(rfc822);
Console.WriteLine("Date: {0}", parsedRFC822);

 

传入DateTime.toString()方法格式符号"r"实际上产生的是RFC1123规范格日期时间串，基于http://www.w3.org/Protocols/rfc822/#z28规范得知,当时传入RFC-822日期也是可以的。
我已经使用该方法创建了RSS源，并且通过了http://validator.w3.org/feed/check.cgi校验器的验证。
该方法的缺点是，在转换中他将日期时间转换成GMT格式的，要转回本地时间，您需要使用本地时区偏移量。为此，您可以使用TimeZone类获取当前时区偏移量，并将GMT替换成时区偏移量时间的字符串。

TimeZone tz = TimeZone.CurrentTimeZone;

String offset = tz.GetUtcOffset().ToString();
// My locale is Mountain time; offset is set to "-07:00:00"
// if local time is behind utc time, offset should start with "-".
// otherwise, add a plus sign to the beginning of the string.
if (!offset.StartsWith("-"))
  offset = "+" + offset; // Add a (+) if it's a UTC+ timezone
offset = offset.Substring(0,6); // only want the first 6 chars.
offset = offset.Replace(":", ""); // remove colons.
// offset now looks something like "-0700".
rfc822 = rfc822.Replace("GMT", offset);
// The rfc822 string can now be parsed back to a DateTime object,
// with the local time accounted for.
DateTime new = DateTime.Parse(rfc822);

 

解决方法2：

这是 C# 中有关如何解析 DateTime 并将其转换为其 RFC-822 表示形式和转换日期时间的实现。
它唯一的限制是DateTime是UTC格式的。我承认这不是很优雅的代码，但他是能运行的。

/// <summary>
/// Provides methods for converting <see cref="DateTime"/> structures 
/// to and from the equivalent <a href="http://www.w3.org/Protocols/rfc822/#z28">RFC 822</a> 
/// string representation.
/// </summary>
public class Rfc822DateTime
{
    //============================================================
    //  Private members
    //============================================================
    #region Private Members
    /// <summary>
    /// Private member to hold array of formats that RFC 822 date-time representations conform to.
    /// </summary>
    private static string[] formats = new string[0];
    /// <summary>
    /// Private member to hold the DateTime format string for representing a DateTime in the RFC 822 format.
    /// </summary>
    private const string format     = "ddd, dd MMM yyyy HH:mm:ss K";
    #endregion

    //============================================================
    //  Public Properties
    //============================================================
    #region Rfc822DateTimeFormat
    /// <summary>
    /// Gets the custom format specifier that may be used to represent a <see cref="DateTime"/> in the RFC 822 format.
    /// </summary>
    /// <value>A <i>DateTime format string</i> that may be used to represent a <see cref="DateTime"/> in the RFC 822 format.</value>
    /// <remarks>
    /// <para>
    /// This method returns a string representation of a <see cref="DateTime"/> that utilizes the time zone 
    /// offset (local differential) to represent the offset from Greenwich mean time in hours and minutes. 
    /// The <see cref="Rfc822DateTimeFormat"/> is a valid date-time format string for use 
    /// in the <see cref="DateTime.ToString(String, IFormatProvider)"/> method.
    /// </para>
    /// <para>
    /// The <a href="http://www.w3.org/Protocols/rfc822/#z28">RFC 822</a> Date and Time specification 
    /// specifies that the year will be represented as a two-digit value, but the 
    /// <a href="http://www.rssboard.org/rss-profile#data-types-datetime">RSS Profile</a> recommends that 
    /// all date-time values should use a four-digit year. The <see cref="Rfc822DateTime"/> class 
    /// follows the RSS Profile recommendation when converting a <see cref="DateTime"/> to the equivalent 
    /// RFC 822 string representation.
    /// </para>
    /// </remarks>
    public static string Rfc822DateTimeFormat
    {
        get
        {
            return format;
        }
    }
    #endregion

    #region Rfc822DateTimePatterns
    /// <summary>
    /// Gets an array of the expected formats for RFC 822 date-time string representations.
    /// </summary>
    /// <value>
    /// An array of the expected formats for RFC 822 date-time string representations 
    /// that may used in the <see cref="DateTime.TryParseExact(String, string[], IFormatProvider, DateTimeStyles, out DateTime)"/> method.
    /// </value>
    /// <remarks>
    /// The array of the expected formats that is returned assumes that the RFC 822 time zone 
    /// is represented as or converted to a local differential representation.
    /// </remarks>
    /// <seealso cref="ConvertZoneToLocalDifferential(String)"/>
    public static string[] Rfc822DateTimePatterns
    {
        get
        {
            if (formats.Length > 0)
            {
                return formats;
            }
            else
            {
                formats = new string[35];

                // two-digit day, four-digit year patterns
                formats[0]  = "ddd',' dd MMM yyyy HH':'mm':'ss'.'fffffff zzzz";
                formats[1]  = "ddd',' dd MMM yyyy HH':'mm':'ss'.'ffffff zzzz";
                formats[2]  = "ddd',' dd MMM yyyy HH':'mm':'ss'.'fffff zzzz";
                formats[3]  = "ddd',' dd MMM yyyy HH':'mm':'ss'.'ffff zzzz";
                formats[4]  = "ddd',' dd MMM yyyy HH':'mm':'ss'.'fff zzzz";
                formats[5]  = "ddd',' dd MMM yyyy HH':'mm':'ss'.'ff zzzz";
                formats[6]  = "ddd',' dd MMM yyyy HH':'mm':'ss'.'f zzzz";
                formats[7]  = "ddd',' dd MMM yyyy HH':'mm':'ss zzzz";

                // two-digit day, two-digit year patterns
                formats[8]  = "ddd',' dd MMM yy HH':'mm':'ss'.'fffffff zzzz";
                formats[9]  = "ddd',' dd MMM yy HH':'mm':'ss'.'ffffff zzzz";
                formats[10] = "ddd',' dd MMM yy HH':'mm':'ss'.'fffff zzzz";
                formats[11] = "ddd',' dd MMM yy HH':'mm':'ss'.'ffff zzzz";
                formats[12] = "ddd',' dd MMM yy HH':'mm':'ss'.'fff zzzz";
                formats[13] = "ddd',' dd MMM yy HH':'mm':'ss'.'ff zzzz";
                formats[14] = "ddd',' dd MMM yy HH':'mm':'ss'.'f zzzz";
                formats[15] = "ddd',' dd MMM yy HH':'mm':'ss zzzz";

                // one-digit day, four-digit year patterns
                formats[16] = "ddd',' d MMM yyyy HH':'mm':'ss'.'fffffff zzzz";
                formats[17] = "ddd',' d MMM yyyy HH':'mm':'ss'.'ffffff zzzz";
                formats[18] = "ddd',' d MMM yyyy HH':'mm':'ss'.'fffff zzzz";
                formats[19] = "ddd',' d MMM yyyy HH':'mm':'ss'.'ffff zzzz";
                formats[20] = "ddd',' d MMM yyyy HH':'mm':'ss'.'fff zzzz";
                formats[21] = "ddd',' d MMM yyyy HH':'mm':'ss'.'ff zzzz";
                formats[22] = "ddd',' d MMM yyyy HH':'mm':'ss'.'f zzzz";
                formats[23] = "ddd',' d MMM yyyy HH':'mm':'ss zzzz";

                // two-digit day, two-digit year patterns
                formats[24] = "ddd',' d MMM yy HH':'mm':'ss'.'fffffff zzzz";
                formats[25] = "ddd',' d MMM yy HH':'mm':'ss'.'ffffff zzzz";
                formats[26] = "ddd',' d MMM yy HH':'mm':'ss'.'fffff zzzz";
                formats[27] = "ddd',' d MMM yy HH':'mm':'ss'.'ffff zzzz";
                formats[28] = "ddd',' d MMM yy HH':'mm':'ss'.'fff zzzz";
                formats[29] = "ddd',' d MMM yy HH':'mm':'ss'.'ff zzzz";
                formats[30] = "ddd',' d MMM yy HH':'mm':'ss'.'f zzzz";
                formats[31] = "ddd',' d MMM yy HH':'mm':'ss zzzz";

                // Fall back patterns
                formats[32] = "yyyy'-'MM'-'dd'T'HH':'mm':'ss'.'fffffffK"; // RoundtripDateTimePattern
                formats[33] = DateTimeFormatInfo.InvariantInfo.UniversalSortableDateTimePattern;
                formats[34] = DateTimeFormatInfo.InvariantInfo.SortableDateTimePattern;

                return formats;
            }
        }
    }
    #endregion

    //============================================================
    //  Public Methods
    //============================================================
    #region Parse(string s)
    /// <summary>
    /// Converts the specified string representation of a date and time to its <see cref="DateTime"/> equivalent.
    /// </summary>
    /// <param name="s">A string containing a date and time to convert.</param>
    /// <returns>
    /// A <see cref="DateTime"/> equivalent to the date and time contained in <paramref name="s"/>, 
    /// expressed as <i>Coordinated Universal Time (UTC)</i>.
    /// </returns>
    /// <remarks>
    /// The string <paramref name="s"/> is parsed using formatting information in the <see cref="DateTimeFormatInfo.InvariantInfo"/> object.
    /// </remarks>
    /// <exception cref="ArgumentNullException"><paramref name="s"/> is a <b>null</b> reference (Nothing in Visual Basic).</exception>
    /// <exception cref="ArgumentNullException"><paramref name="s"/> is an empty string.</exception>
    /// <exception cref="FormatException"><paramref name="s"/> does not contain a valid RFC 822 string representation of a date and time.</exception>
    public static DateTime Parse(string s)
    {
        //------------------------------------------------------------
        //  Validate parameter
        //------------------------------------------------------------
        if (String.IsNullOrEmpty(s))
        {
          throw new ArgumentNullException("s");
        }

        DateTime result;
        if (Rfc822DateTime.TryParse(s, out result))
        {
            return result;
        }
        else
        {
            throw new FormatException(String.Format(null, "{0} is not a valid RFC 822 string representation of a date and time.", s));
        }
    }
    #endregion

    #region ConvertZoneToLocalDifferential(string s)
    /// <summary>
    /// Converts the time zone component of an RFC 822 date and time string representation to its local differential (time zone offset).
    /// </summary>
    /// <param name="s">A string containing an RFC 822 date and time to convert.</param>
    /// <returns>A date and time string that uses local differential to describe the time zone equivalent to the date and time contained in <paramref name="s"/>.</returns>
    /// <exception cref="ArgumentNullException"><paramref name="s"/> is a <b>null</b> reference (Nothing in Visual Basic).</exception>
    /// <exception cref="ArgumentNullException"><paramref name="s"/> is an empty string.</exception>
    public static string ConvertZoneToLocalDifferential(string s)
    {
        string zoneRepresentedAsLocalDifferential   = String.Empty;

        //------------------------------------------------------------
        //  Validate parameter
        //------------------------------------------------------------
        if (String.IsNullOrEmpty(s))
        {
          throw new ArgumentNullException("s");
        }

        if(s.EndsWith(" UT", StringComparison.OrdinalIgnoreCase))
        {
            zoneRepresentedAsLocalDifferential  = String.Concat(s.Substring(0, (s.LastIndexOf(" UT") + 1) ), "+00:00");
        }
        else if (s.EndsWith(" GMT", StringComparison.OrdinalIgnoreCase))
        {
            zoneRepresentedAsLocalDifferential  = String.Concat(s.Substring(0, (s.LastIndexOf(" GMT") + 1 ) ), "+00:00");
        }
        else if (s.EndsWith(" EST", StringComparison.OrdinalIgnoreCase))
        {
            zoneRepresentedAsLocalDifferential  = String.Concat(s.Substring(0, (s.LastIndexOf(" EST") + 1)), "-05:00");
        }
        else if (s.EndsWith(" EDT", StringComparison.OrdinalIgnoreCase))
        {
            zoneRepresentedAsLocalDifferential  = String.Concat(s.Substring(0, (s.LastIndexOf(" EDT") + 1)), "-04:00");
        }
        else if (s.EndsWith(" CST", StringComparison.OrdinalIgnoreCase))
        {
            zoneRepresentedAsLocalDifferential  = String.Concat(s.Substring(0, (s.LastIndexOf(" CST") + 1)), "-06:00");
        }
        else if (s.EndsWith(" CDT", StringComparison.OrdinalIgnoreCase))
        {
            zoneRepresentedAsLocalDifferential  = String.Concat(s.Substring(0, (s.LastIndexOf(" CDT") + 1)), "-05:00");
        }
        else if (s.EndsWith(" MST", StringComparison.OrdinalIgnoreCase))
        {
            zoneRepresentedAsLocalDifferential  = String.Concat(s.Substring(0, (s.LastIndexOf(" MST") + 1)), "-07:00");
        }
        else if (s.EndsWith(" MDT", StringComparison.OrdinalIgnoreCase))
        {
            zoneRepresentedAsLocalDifferential  = String.Concat(s.Substring(0, (s.LastIndexOf(" MDT") + 1)), "-06:00");
        }
        else if (s.EndsWith(" PST", StringComparison.OrdinalIgnoreCase))
        {
            zoneRepresentedAsLocalDifferential  = String.Concat(s.Substring(0, (s.LastIndexOf(" PST") + 1)), "-08:00");
        }
        else if (s.EndsWith(" PDT", StringComparison.OrdinalIgnoreCase))
        {
            zoneRepresentedAsLocalDifferential  = String.Concat(s.Substring(0, (s.LastIndexOf(" PDT") + 1)), "-07:00");
        }
        else if (s.EndsWith(" Z", StringComparison.OrdinalIgnoreCase))
        {
            zoneRepresentedAsLocalDifferential  = String.Concat(s.Substring(0, (s.LastIndexOf(" Z") + 1)), "+00:00");
        }
        else if (s.EndsWith(" A", StringComparison.OrdinalIgnoreCase))
        {
            zoneRepresentedAsLocalDifferential  = String.Concat(s.Substring(0, (s.LastIndexOf(" A") + 1)), "-01:00");
        }
        else if (s.EndsWith(" M", StringComparison.OrdinalIgnoreCase))
        {
            zoneRepresentedAsLocalDifferential  = String.Concat(s.Substring(0, (s.LastIndexOf(" M") + 1)), "-12:00");
        }
        else if (s.EndsWith(" N", StringComparison.OrdinalIgnoreCase))
        {
            zoneRepresentedAsLocalDifferential  = String.Concat(s.Substring(0, (s.LastIndexOf(" N") + 1)), "+01:00");
        }
        else if (s.EndsWith(" Y", StringComparison.OrdinalIgnoreCase))
        {
            zoneRepresentedAsLocalDifferential  = String.Concat(s.Substring(0, (s.LastIndexOf(" Y") + 1)), "+12:00");
        }
        else
        {
            zoneRepresentedAsLocalDifferential  = s;
        }

        return zoneRepresentedAsLocalDifferential;
    }
    #endregion

    #region ToString(DateTime utcDateTime)
    /// <summary>
    /// Converts the value of the specified <see cref="DateTime"/> object to its equivalent string representation.
    /// </summary>
    /// <param name="utcDateTime">The Coordinated Universal Time (UTC) <see cref="DateTime"/> to convert.</param>
    /// <returns>A RFC 822 string representation of the value of the <paramref name="utcDateTime"/>.</returns>
    /// <exception cref="ArgumentException">The specified <paramref name="utcDateTime"/> object does not represent a <see cref="DateTimeKind.Utc">Coordinated Universal Time (UTC)</see> value.</exception>
    public static string ToString(DateTime utcDateTime)
    {
        if (utcDateTime.Kind != DateTimeKind.Utc)
        {
            throw new ArgumentException("utcDateTime");
        }

        return utcDateTime.ToString(Rfc822DateTime.Rfc822DateTimeFormat, DateTimeFormatInfo.InvariantInfo);
    }
    #endregion

    #region TryParse(string s, out DateTime result)
    /// <summary>
    /// Converts the specified string representation of a date and time to its <see cref="DateTime"/> equivalent.
    /// </summary>
    /// <param name="s">A string containing a date and time to convert.</param>
    /// <param name="result">
    /// When this method returns, contains the <see cref="DateTime"/> value equivalent to the date and time 
    /// contained in <paramref name="s"/>, expressed as <i>Coordinated Universal Time (UTC)</i>, 
    /// if the conversion succeeded, or <see cref="DateTime.MinValue">MinValue</see> if the conversion failed. 
    /// The conversion fails if the s parameter is a <b>null</b> reference (Nothing in Visual Basic), 
    /// or does not contain a valid string representation of a date and time. 
    /// This parameter is passed uninitialized.
    /// </param>
    /// <returns><b>true</b> if the <paramref name="s"/> parameter was converted successfully; otherwise, <b>false</b>.</returns>
    /// <remarks>
    /// The string <paramref name="s"/> is parsed using formatting information in the <see cref="DateTimeFormatInfo.InvariantInfo"/> object. 
    /// </remarks>
    public static bool TryParse(string s, out DateTime result)
    {
        //------------------------------------------------------------
        //  Attempt to convert string representation
        //------------------------------------------------------------
        bool wasConverted   = false;
        result              = DateTime.MinValue;

        if (!String.IsNullOrEmpty(s))
        {
            DateTime parseResult;
            if (DateTime.TryParseExact(Rfc822DateTime.ConvertZoneToLocalDifferential(s), Rfc822DateTime.Rfc822DateTimePatterns, DateTimeFormatInfo.InvariantInfo, DateTimeStyles.AdjustToUniversal, out parseResult))
            {
                result          = DateTime.SpecifyKind(parseResult, DateTimeKind.Utc);
                wasConverted    = true;
            }
        }

        return wasConverted;
    }
    #endregion
}

 评论：RFC822日期规范中，天和秒是可选的，以上代码没有处理。