How do I parse and convert DateTime’s to the RFC 822 date-time format?

Question

How do I convert a DateTime structure to its equivalent RFC 822 date-time formatted string representation and parse this string representation back to a DateTime structure? The RFC-822 date-time format is used in a number of specifications such as the RSS Syndication Format.

Answer 1

This is an implementation in C# of how to parse and convert a DateTime to and from its RFC-822 representation. The only restriction it has is that the DateTime is in Coordinated Universal Time (UTC).

/// <summary>
/// Provides methods for converting <see cref="DateTime"/> structures 
/// to and from the equivalent <a href="http://www.w3.org/Protocols/rfc822/#z28"&gt;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"&gt;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"&gt;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
        //------------------------------------------------------------
        Guard.ArgumentNotNullOrEmptyString(s, "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
        //------------------------------------------------------------
        Guard.ArgumentNotNullOrEmptyString(s, "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
}

Answer 2

Try this:

  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);

The "r" format specifier passed into DateTime's ToString() method actually yields an RFC-1123-formatted datetime string, but passes as an RFC-822 date as well, based on reading the specification found at http://www.w3.org/Protocols/rfc822/#z28. I've used this method in creating RSS feeds, and they pass validation based on the validator available at http://validator.w3.org/feed/check.cgi.

The downside is that, in the conversion, it converts the datetime to GMT. To convert back to local time you would need to apply your local timezone offset. For that, you might use the TimeZone class to get your current timezone offset, and replace "GMT" with a timezone offset string:

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);

Answer 3

The only problem (which is a big one) with that approach is the "r" format doesnt recognise any other timezone code except GMT so to parse other timezones you have to do some manual conversion to a numeric value before parsing. I really hope .NET adds support for timezone codes sometime in the future

Answer 4

Here is how Microsoft does it in the Rss20FeedFormatter. Oppositional's code doesn't get rid of the ":" in the GMT offset portion. Jeff Woodman's appears to do this. The code below does this as well (if not using Atom10FeedFormatter.zeroOffset).

private string AsString(DateTimeOffset dateTime)
{
    if (dateTime.Offset == Atom10FeedFormatter.zeroOffset)
    {
        return dateTime.ToUniversalTime().ToString("ddd, dd MMM yyyy HH:mm:ss Z", CultureInfo.InvariantCulture);
    }
    StringBuilder builder = new StringBuilder(dateTime.T)oString("ddd, dd MMM yyyy HH:mm:ss zzz", CultureInfo.InvariantCulture));
    builder.Remove(builder.Length - 3, 1);
    return builder.ToString();
}